I usually change when I get to work because my commute is long enough to sweat. But if I ride or do an errand at lunch I'll be wearing jeans and have experienced what you're describing. I stick with Levi's and noticed that a couple of my pairs were of the "stretch" variety. Not tight, same fit as the regular fabric, but with a little stretch I notice when throwing a leg over the saddle and in the thighs/knees.

So you might try a stretch fabric next time you're jeans shopping. I really haven't noticed any difference in durability or fit with the Levi's stretch; other brands might do something like that as well.