Rick writes, "A systematic survey of the size and disposition of church choirs in western Europe during the fifteenth and sixteenth centuries, Brown states, "is likely to show that sacred polyphony in the fifteenth century was normally sung by relatively small groups, often with only one singer to a part, that choir sizes increased suddenly at the end of the fifteenth century or in the early sixteenth century (when fifteen to twenty-five singers appear to have constituted a large choir), and that there was a tendency to use more singers on the top and bottom lines than on the inner parts.""
This late 15th century increase in choir size would accord with EdwardBast's mention of the 1483 Papal bull that set the Sistine Chapel choir at 24 singers. However, it may also present us with similar tricky territory to Joshua Rifkin's argument for OVPP choirs in the Baroque era. To borrow from Rifken's 'baseball analogy'--do the "15 to 24" singers represent all of the singers used to sing a weekly mass in the Sistine chapel, or rather is that size representative of a "full team roster"--i.e., the bullpen & the dugout, from which a baseball manager can choose his starting line up of 9 players each week? Was "the doubling", as Phil writes, "more to insure at least a singer per part in attendance in case of one being sick rather than for the actual sound"?
If it can be proven that the size of choirs singing masses increased to 15 to 25 singers after 1483, under the influence of this papal bull, then it would suggest that, yes, many of the sacred works by composers in Rome during the latter part of the 15th and early part of the 16th century were likely composed with larger choirs in mind (motets probably included).
But, of course, we don't know how the various composers of the day, who were evidently accustomed to writing choral music for smaller choirs, felt about the papal bull of 1483--whether they liked it or not, or whether the papal bull was enforced throughout Italy & beyond, afterwards, by subsequent popes (especially since Sixtus IV died not longer after in 1484). Though, granted, from what Rick writes, it does appear that there was a significant effort made to conform to the papal bull.
But, if the choirs at Cambrai cathedral were never more than two singers on a part during Dufay's years there, that also suggests that prior to 1483 large choirs were never the norm, and possibly seldom even used in the churches of the Burgundian and Franco-Flemish composers in the north, where most of the major composers of the Renaissance were born and spent their formative years. Indeed by the time the Burgundian & Franco-Flemish composers attained posts in Italy (Dufay, Josquin, Obrecht, etc.) they were already highly regarded composers, if not renowned across Europe. That isn't insignificant.
Presumably, Dufay's Cambrai cathedral was well funded--being that the Dukes of Burgundy were important patrons during the early Renaissance. Hence, Dufay's consistent decision to never use more than 2 singers on a part in his masses at Cambrai may not have been due to a lack of funds, but rather an aesthetic choice. Given that the Sistine Chapel was likewise probably not lacking in funds, it may well be that what we're seeing here is evidence of two conflicting aesthetics regarding the size of choirs during the early Renaissance: one that was influenced by the Burgundian school and French, Flemish, Burgundian, & Hapsburg nobility in the north (which influenced all the noble courts of Europe, including ones in Italy), and another which came directly under the influence of the Vatican in the years leading up to the papal bull of 1483 and afterwards.
I should also point out that the other hugely influential composer of the early & middle Renaissance, Josquin Desprez, didn't arrive in Rome until 1489: when he became part of the papal choir and composed there until 1495--a period of six years. Indeed, that is the time when Josquin is thought to have scratched his name on the wall of the Sistine chapels's choir room. While it is most likely that Josquin's compositions during this period do reflect a willingness to conform to the 1483 papal bull and compose for a larger choir, his Roman years were hardly a 'formative' period in Josquin's career, given that his birth date is generally supposed to be around 1450. In other words, Josquin was nearly 50 years old when he arrived in Rome in 1489. So, he'd been a mature composer for several decades.
Indeed Josquin's most critical formative years were spent elsewhere--first as a choirboy (along with composer Jean Mouton) in the royal church of Saint-Quentin (around 1460)--Saint-Quentin being a disputed city between the King of France and the dukes of Burgundy in the 15th century; then he is thought to have studied counterpoint with Johannes Ockeghem (who is thought to have learned his counterpoint from the Burgundian composer, Gilles Binchois); then Josquin became a singer/composer in the employ of René, the Duke of Anjou, in Aix-en-Provence (1470s), René being one of the most influential figures of the Renaissance; followed by a possible subsequent connection to King Louis XI in Paris (with René's death in 1480), then back to his home town of Condé-sur l'Escaut in 1483 where his family is thought to have been killed in a siege by Louis XI, and then into the service of the Sforza family in Ferrara or Naples, and to Milan in 1483 or 84 where the Sforza family returned from exile, and subsequently to Rome in 1489 (though Josquin may have made an earlier visit to Rome while in the service of the Sforza family in the early 1480s).
All of which shows that Josquin's early and middle years as a composer were spent under the influence of such major Renaissance figures such as Johannes Ockeghem and René d' Anjou, etc., prior to the 1483 papal bull & his six years at the Sistine chapel. Therefore, if Dufay's 'northern' practices at Cambrai Cathedral are not an anomaly, but rather indicative of the musical practices of the composers of the Burgundian & Franco-Flemish schools, then Josquin compositions prior to his arrival in Rome in 1489 were most likely closely tailored to Dufay's practices at Cambrai, and were not composed under any influence from Pope Sixtus IV in Rome.
Furthermore, Josquin's compositions and techniques are not all the same, but vary throughout his career. Some of his works are written in a more austere, unadorned manner--without ornamentation (& therefore, were possibly composed with the expectation that they would be sung by larger choirs), while other works require considerably more virtuosity from the singers (& therefore, were possibly composed with smaller choirs in mind). The one work that we can be virtually certain that Josquin didn't compose for a small choir is his 36 part motet "Qui Habitat", and not surprisingly that composition is different from most of his other motets. I'd be surprised if "Qui Habitat", with its massive wall of sound, wasn't composed while Josquin was under the spell of Rome (though I don't know if the motet has ever been dated):
I should also add that if the book Phil refers to--"Renaissance Music", by Kenneth Kreitner-- is correct that "OVPP was common in Palestrina's day", then the papal bull of 1483 wasn't still in effect by the time of Palestrina, who is viewed as the major composer of the late Renaissance (along with Lassus). In other words, if OVPP was common in Italy in Palestrina's day, that would mean that one or more of the subsequent popes after Sixtus IV had either changed things back to the way they were before the papal bull of 1483, or that the papacy had become more lenient, and possibly even sympathetic towards composers regarding how they wished to hear their music sung by choirs?
Or, it's also possible, I suppose, that Joshua Rifkin's 'baseball analogy' holds true for the size of choirs throughout the early, middle, and late Renaissance too (in addition to the Baroque period), and therefore the papal bull of 1483 was more about increasing the size of available chorus members (to draw upon), rather than the size and sound of the choir itself, in the singing of weekly masses in Catholic churches.