You are so far off with your Tri training comment. The Diaz brothers are both very good in spite of that training, not because of it. Their skill and grit mask areas they could certainly improve on. The Tri training is not bad as some off-season cross training but you'd be fooling yourself to think it is the cornerstone for a successful MMA career. This is dark ages thinking. If anything, it is simply the Diaz Bros gravitating to a sport which suits their specific body make-up and mentality.
No weight training. Guess you missed the whole Bruce Lee's dispelling of that over 40 years ago....and just about every fighter since. You think Frankie Edgar's motor has nothing to do with heavy weight training such as deadlifts, etc?
Finally, to the OP, there should be a caveat as to what type of skinny 140 lb. guy we're talking about, a skinny runner, or a skinny powerlifter, or wrestling-type; both might appear skinny but their training has a huge difference on the nervous system. Too many people focus on the muscular and cardiovasc. system with zero regard for the training and the body's nervous system response. It's why many folks seem to be "wired" differently. Usually some meshing of natural inclination and specific training.