Depending on the Boots and how they are stored, they can shrink as well. I have a pair of Nick's loggers that are 15 years old, full leather. If I go for a few months without wearing them, I have to oil them, heat them, and wear them around the house for a bit to stretch the leather back out. I also had a pair of full leather Hanwags that I didn't oil after hunting season, they sat in my dry mechanical room all year, and seemed to shrink more than one full size. I never got them stretched back out. So in short, it could be your foot like said by others, but in my experience, it could also be the boot.