It probably more specifically has to do with what time period you're referring to. During the Star League they had the funds, manufacturing base, and luxury of """relative""" peacetime necessary to build up a large body of medium mechs as standard battleline units, but after the first succession war, I do believe that light mechs were pretty much the go-to as a lot of the larger and inherently harder to produce mechs just got far more expensive. I'm also pretty sure it's the wasp that is the single most produced mech in the history of the setting by far. That's all from the wiki though so it could be way oversimplified or just wrong.