Ah yes the whole issue of trouser fitting and the rarely-mentioned Lady Lunch. Quite apart from patterns never being drafted to accommodate bums, bellies and hips, they DEFINITELY never allow room for a generous foo!
I have a flat bum, but plenty of belly and lunch, the missus has a teeny pot tum, and a curvaceous bum. We did that goldfish-bowl crotch curve thing with the rolled up tin foil, and it's really interesting [go seek it out on my blog or youtube, it's there somewhere]
It's a good place to start if you know that the crotch curve is where all your problems lie, you can at least lie it on a new pattern and make some initial tweaks to reduce the toile stages.