I have lately seen often claims that on 25 man bosses hit much harder than their 10 man counterparts. And while they do hit harder, some players have gone that far as to state that on 25 man they hit at least +50% if not twice as hard. Compared to this, most abilities do 20-30% more damage, and according to logs on WoL even their melee swings are within the +25% limit.
Which is completely fine IMO. Since with different gearing tanks can reach that +30% in health pool.

But where do these claims come from then? From the era when 10 man drooped weaker loot than 25 man?

More specifically, this was brought up in some discussions with the intended chance to vengeance caps. [The new cap would be 30% of HP on 10man, and 50% on 25 man.] While many people think that 40%/50% would be more realistic (and it is within the +20-30%), quite some 25man tanks think that they need the 67% higher cap. And this doesn't even calculates in their different gearing strategies, which inflates this even more.

Or are 25man tanks simply looking down on their 10man counterparts this way? Is this a part of the endless 25vs10 debate?

Any opinions?