Goofy (the cocker) has something similar to that on her back paw.
Doesn't seem to bug her either, except if the groomer nicks it cleaning the hair.

Our vet said the same thing, he says it is a fatty growth and to keep an eye on it. It seems to swell in the summer a bit and shrink during the colder months.
Ashamed to admit it but I forget to check it more often than not.