It can be hard to gauge sometimes. for example porier moved up, probably did better at LW but never won the title much like his time at FW so who is to say it really helped him? hell maybe he'd have beaten max at FW if he could have gotten there. RDA is another one, he found many successes at WW but never made it to the top, he was probably better off at LW
meanwhile guys like Edgar and aldo both had great success in moving down though neither won titles at the lower weights. I think it's reasonable to say frankie was better fit at FW than LW but he was a champ at LW. Aldo probably better at FW but it's hard to tell because he only loses to the best fighters (Yan, Max, Volk)
Cannonier moved down 3 weight classes (HW, LHW, MW) and got a title shot. same with florian (WW, LW, FW)
Different strokes for different folks. Most people cant move down weight classes to begin with because they cut so much to be in the category they are already in. I think that's why you see more success with moving up, it's easier to do than dropping a class, it's more likely that the fighter can.