There appears to exist a common problem with the fit of trousers: They are either to wide or to tight at the crotch.
To get a solution you only need to know the following, which is merely a reproduction of an article in a very old (1920s) German cutting manual, which is no longer in my posession and now somewhere in the UK.
Have you ever wondered why jeans, that fit so closely at the legs yet are so comfortable at seat and crotch level? Or why those wide legged trousers feel so uncomfortable at the same area?
The reason is quite simple: The slimmer the legs, the more the crotch opens. This is a diagram I made (using very simple techniques) to demonstrate this (note that the front and rear crotch size stays the same).
This would represent a pair of trousers with a larger knee width. As you can see, the distance from A to B is relatively small compared to the version with a small knee width:
This would represent a pair of trousers with a smaller knee width. As you can see, the distance from A to B is relatively wider compared to the version with a wider knee width:
What this basically means is that for the wider knee width you need a wider crotch diameter and vice versa to have the same comfort.