I learned a few tricks from wearing my diapers last weekend...
1) I'm sure you've heard of the X position (where the tapes are arranged so the top and bottom ones face the center) It really does work, and provides a somewhat better fit.
2) Are those jeans too tight? That can keep your padding from expanding, so all that fluid has only one other place to go: down your legs.
Is there is fix? No need to burn your money on larger pants. Just take 2 rubber bands, put them around the button, through the hole and back around. This will allow your pants to be less constrictive. Just be sure you have a long shirt or coat to hide that open zipper.