Which differences would one expect? Which particular proportion and why? How to get estimates for something like this? I have no idea.I didn't say "high in relation to the number of men". There were more male composers, but the difference is not as big as one would expect. The number of female composers was high compared to today's expectations and higher than in other areas at the time like politics.
One could argue that compared to many other fields there were a lot of women active in classical music in the 18th and 19th century, including professional performers. So from this fact one might expect more female composers than there were, or more famous ones. OTOH, I don't think any female composer of the 19th century is today regarded as highly as e.g. Jane Austen, the Bronte sisters and a few other female writers, *despite* a lot of female performers and high hurdles for female writers (like publishing under male pseudonyms etc.).
Is this selective sexism against Fanny Mendelssohn or an justified estimation that the historical and artistic importance of e.g. "Wuthering Heights" is on a different level than Fanny's chamber music or lieder?
Sexism is one factor, but it often seems a lazy catch-all (non-)explanation, if suggested as the only or main factor. It also works in strange ways. Why do we find female conductors still uncommon when we had female star performers since more than 200 years? If we were such sexists why didn't we ban the opera divas or Clara Schumann from performing?