It is equivalent. Tanks don't do 0 damage. If we consider tanks to be half a DPS each, then you have 28500000/18 = 1.583m dmg each, and 9200000/6 = 1.583m dmg each. In this case then, it is easier to do it 10 man, mainly because it is easier to class stack. If I have a 10 man raid with a rogue, a mage, and a shadow priest, that's equivalent to having 10 rogue/mage/shadow priests in 25-man, out of 17 dps. There are 10 man raids that are completely stacked for this fight, yet don't even look out of the ordinary - Rogue, Mage, SPriest, Ele, Ret. Can you imagine a 25 man raid with only those 5 DPS classes?