TS is right. If anything MMA, combat sports in general should've proven that your figure isn't a good indicator of how good a fighter you are. I'm going to go even further than TS and expand to all sorts of body types that people look down on that were successful nonetheless.
Examples: Frankie Edgar is "small" for LW but is arguably the best LW ever.
BJ Penn, the other possibly best LW ever, also happened to be a WW champ. Regardless of whether or not you think he fought best here or there he was a WW champ, so at one point in time he was the best WW regardless of how "small" he was.
Shogun, Evans, Machida, Hendo were all LHW champs and no matter that some fans claim they should cut weight cuz they seem a little small, and in Shogun's case supposedly carry around a little extra chub, they were all legit champs.
Fedor, the greatest HW still was supposedly undersized and chubby.
Tim Sylvia, believe it or not so far the most dominant UFC champ, was fat clumsy but he still was definitely a dominant champ at his peak.
In fact I think when guys supposedly "look" better at a lower division after they've shed that extra chub or been pressured to reboot their career by fighting at a supposedly more natural weight division it's all just hype.
Rich Franklin is the only example I can think of a guy who purposely cut weight to improve his chances of getting a belt who did in fact get a belt by dropping a division.