Your stirrup leather can stretch both from mounting your horse and from uneven pressure.  Most likely though if your stirrups are even you have tight muscles on one side that isn't allowing full extenion of your leg.  Have you tried utlizing some leg and lower back stretches prior to riding to see if that made things easier?

My hips are off kilter which could be a lot of it. I do a lot of leg stretches 3x a week after spin class. I could also ask my chiro to make sure to do a specific adjustment that helps when my hips get all put of whack. That may help too!