Normally when a snake swallows a rodent head-first, the feet (front or back) are flexible and can be swallowed easily.
The only thing I can think of besides the rat being WAY too big is if the rodent was dead a while before freezing, so that rigor mortis made the body stiff...thus, even when thawed the legs & feet were not flexible.
You perhaps need a better supplier of rodents, IF this was the case...as rodents dead long enough to get stiff have started to spoil prior to freezing. And that
is unacceptable.