Cotto is not too small for 154. He just needs to be more defensively sound at this stage of his career. He could get away with slugging it out with guys @ 140 and 147. Not so much @ 154...
Cotto hasn't beaten a notable fighter at 154. His only claim to fame is beating a 1 eyed Margo, a 1 legged rabbi, and a 1 good win Mayorga.
Cotto maximized his potential. He was never going to be dominant wherever he fought. If he had faced Mayweather at 147, he would have been beaten worse. And it may well be that Trout is the next Winky Wright. We'll see.
People see his frame and assume that he should be able to make 147 without factoring in his lifestyle and his age.