Heh... Fatboy is also the name of a packaged-in mod for the (now) old FPS Unreal Tournament. It tweaks the gameplay in the same way paprika tweaks a good salsa: subtly, yet thoroughly.
This mod doesn't change the weapons. It leaves the arenas alone. The game will start off without suspicion until someone scores the first frag. When that happens, they grow.
Well, 'grow' may not be the right word, unless you count growing outward. For you see, Fatboy increases your character model's thickness in proportion with how many kills you've scored in-game. See that fatso rounding the corner with the rocket launcher? He's likely one of the more elite players on the server right now. Maybe you'd be better off picking on the newly respawned chap in the other corridor... if you can hit him, that is, while he's the size of a toothpick.
At the end of every round in Unreal Tournament, the top score list displayed and everyone got to see the lead player frozen in position while the next map loaded. Usually the top fragger busted out a few of the built-in taunts, gestures, and hip-pumping bravado. Only in this case, it was being done by what looked to be a roughly human-shaped blimp wearing a UT model costume.
Despite the great bizarre feeling of shooting up inflated warriors, not many online servers used Fatboy, probably because it served as a tactical handicap. To my knowledge, there was no conversion of Fatboy to Unreal Tournament 2003. Which really is a shame.