Darby and the Bucks fight like you want a smaller guys fighting bigger ones - relentlessly and with speed. You know, like MMA and Boxing when you put the smaller and faster guy in a bigger division, like Roy Jones Jr against Ruiz and etc. Also, they are as tall as FTR and Matt Jackson right now isn't that slim to begin with, his muscles might not be defined, but he is on the bulkier side compared with Nick.
I think people see muscles and think "oh yeah this is guys is so strong", but going by logic and real life, big bulky muscles like a lot of wrestlers have per se don't do nothing a lot of times neither are that good to fight, on contrary.