More mobs = more damage per mob
I try to tank Otters with my Crusader. I've got 51 def, Raydric and Orc Warrior Cards. According to calcx.wushuang.ws, with this setup I shouldn't receive more than 192 damage per Otter and it works the same in game. However, when I tank more than 1 Otter, their damage increases - with about 20 Otters I get hit for 500+ per mob! This damage seems to increase gradually - the more monsters, the harder they hit. This doesn't seem to affect SB Monks.