I'll admit that this seems a silly question, but after observing the images from this recent question I had the obvious realization that other than skin tone and dress, Boss Nass looks absolutely nothing like other Gungans! He seems to be missing the tell-tale long ears, his eyes are inset rather than protruded and, unless my recollection is off, not a single other Gungan is overweight.
Of course, the weight can be explained away by simply claiming he's the leader and, therefore, has free-reign to all resources available, especially food. However, even the rest of the Gungan Council at the beginning of the Phantom Menace doesn't appear to have such weight issues which, in my opinion, makes Boss Nass's form all the more odd.
I considered writing off this question but being that there is precedent within the Star Wars universe of creatures setting up other beings not of their species as leaders, it didn't seem to off base of an assumption.