I've been reading a book called "Opera, A History" by Headington, Westbrook and Barfoot.
They tell us that Mozart wrote Mitridate for Milan and that this explains its completely conventional makeup. I think that's why it's a disappointment: he never steps out of the frame. We have come to expect more from Mozart. Idomeneo was Mozart's 13th work for the stage.
They also discuss castrati, saying that they sang most of the parts for both genders. That they were larger than women is easy to imagine, but the exaggeration of their capabilities is a bit hard to buy. They were specifically trained for their professions to a degree that it is hard to imagine would be true for women at the time.
You don't think I should read about things before I write about them, surely! I would like to read more about the true influence of Gluck's reforms on French opera and the transition to the romantic operas of Weber. I think like most transition periods, the leaders of change are not well known to us. I always remember how much Berlioz loved Spohr.