I have this problem too! double jointed at the knees makes for extra large calves :0
Before you start cutting them up, try to relax the muscle by pointing your toes while you zip it up, that might give you enough room to zip it the rest of the way.. wearing them will stretch them.
you can also stretch them if you have something you can stuff into it.. balls? wooden / metal rings? newspaper?
you an also stretch shoes by filling them with water [in bags of course] and putting them in the freezer, the water will expand as it freezes. might not work as well for boots, but if you're from a cold place you might be able to try that by sticking them outside.