Hm, I'm currently at rep 13 and I see plenty of mediums and heavies and quite a few assaults too in random drops.
Sure, warzone missions usually start light with Locusts, Assassins and the ever present Cicadas (yeah, might wanna tone that one down a bit - though it is a good money-maker as salvage) as well as Harassers and Scorpions, but later waves feature Hunchies, Centurions, Griffins and Wolverines all the way up to late-wave Catapults, Archers, Marauders, Awesomes and Cycolops' along with Partisans and Manticores (as well as LRM/SRM 40-60s) and the occasional Demolisher
Rep-10+ assassination targets usually are in heavies/assaults (unless it is an entire lance you have to kill, in which case there is usually a light, a medium, a heavy and an assault or a second heavy) with mediums/heavies as "escorts".
Defense is the same as warzone, with lights in the first one or two waves and drop-weight of hostiles increasing towards the end, sometimes by a _lot_.
Or to quote Baradul: "This is a quick an easy mission" - only to run into a group of 6 assaults and a couple of Partisans
Is there the occasional mission which turns out to be a walk in the park due to rather light opposition?
Sure.
So I caught them with their pants down, what's the problem with that?
P.s. I agree that more difficult does not mean more enemies but harder enemies. Of course, for me, this means enemies that are _better_ not just heavier, i.e. better A.I. - facing the Black Widow Company should be a _vastly_ different experience than facing a run-off-the-mill merc company with the same mechs.
Yeah, I know, dream on, Hawkeye.