This species is hit and miss with respect to eating. Many waste away to nothing, but those that do start eating, indeed, will eat themselves so full they look like they're about to burst. I found that when I had mine I had that problem too and there were times I was *convinced* it was abnormal but it seems it just happens sometimes. Here are some pictures:
(Sorry for the picture quality..)
So .. consider yourself lucky that yours is eating.
Not that I can say with 100% certainty that the "marble with a tail" thing is nothing to worry about, but it does seem that it's at least not completely unheard of.
Here are some pictures of some lawnmower blennies I found in Hawaii (or at least a species very similar, I couldn't get close enough to tell for sure what they were, these were in a tidepool of sorts): (You can just sorta see some fat bellies on these guys..)
