MMA generally has larger gaps in weight classes vs. boxing and wrestling. Gregor is undersized for 155 and too big for 145. As a college athlete, he wrestled at 149, so at age 34 he isn't going to weigh less than he did in his late teens and early 20's. In a perfect world, the UFC would have a couple more weight classes, perhaps 135, 145, 155, 165, 175, 185, 200, 220 and HW. This way the big LW fighters who cut massive amounts of weight could fight at 165 and same thing for the big WW fighters. They could fight at 175 and not have to move up to MW at 185 and go against MW guys who cut from 215 to 225 walk around weight.
RDA lost the LW belt because he could no longer make weight safely. He almost died prior to the Alvarez fight, so not surprising he lost that fight and then lost the next fight vs. Ferguson (eye pokes didn't help either) and then spent several years as a short and small WW going against far big guys like Usman (loss) and Lawler (win).