I thought the first kill had 10 dps, 3 tanks, and 3 healers. I did the math and 8 man and 16 man both required the same DPS per person (or at least extremely close) assuming the enrage timers were the same. If the enrage timer was different on 8 man and 16 man then it makes sense that it took them such a long time to beat it.
Damage comparison between 16 and 8 man mode:
3,265,395 HP / 10 = 326,539 damage per DPS
1,288,301 HP / 4 = 322,075 damage per DPS
Both of these assume the tanks aren't doing any damage. Tank damage should be slightly higher in 16man because they occasionally have two tanks on him at once don't they? Either way, it seems it would be balanced for them both to have the same enrage timer, which it sounds like they do now.
Oooh, you're right. I counted a DPS as a healer. So yeah, it's balanced correctly now