It may be related to the squirrel and marmot, but a groundhog can get to be big animal. The jrank organization says:


The groundhog is a rather large marmot, typically weighing about 6.6-13.2 lb (3-6 kg). One captive animal, however, managed to achieve a enormous 37.4 lb (17 kg) just prior to its wintertime hibernation, when these animals are at their heaviest.

30 years ago, my husband and I were having a picnic on the sloping side of a country cemetery in Kentucky. Suddenly the long grass rustled, and a BIG groundhog waddled out of it, right up to us. We didn't move a muscle, and it gave a startled, affronted snort and went back the way it had come. It seems to me that its body was as about as big as a basketball - that's pretty big.