Yeah, it scaled really funny like that. I did a lot of testing when I did my dungeon and the ShadowKnight merc just melted everything for easy xp. So a group size of four: three pc and the merc. This upgraded the mobs from solo to group and bumped the xp gain by quite a bit, from 300xp a kill solo to all three getting 421 for the same difficulty.

XP actually dropped after that if I had a full group, but I also ran into what you're experiencing and the mobs all becoming raid level and they just one-shot everything.