What is the squishy "sack" on the back of my cat's hind leg?

I just noticed that my cat is "squishy" behind one of his back legs. He is a Maine Coon cat, so he is very fluffy esp in the back leg/tail area, so I can't really tell if there is an actual sack there. But I don't think so. I felt both back legs at the same time, it the leg I'm referring to didn't seem to be bigger than the other, just the flesh was squishy. I didn't press on it, but when touching it, my cat made no reaction of pain or anything. He had a pretty bad back leg injury several yrs ago, and he is 10, so he does have a bit of what I thought was arthitus- if he lands the wrong way after jumping, for instance, he is in pain for a few mins. But in general he is not in pain, these instances are few and far between. What could the squishy-ness be? Just some more info- he has hyper thyroid (I don't think that matters at all- but just some more info)