25men coordination > 10men coordination.
For 25men you 15 awesome dpsers, for 10men you need 5.
For 25men you need 6-7 awesome healers, for 10men you need 2-3.
Every single healer CAN'T fail healing someone, or the raid will mostly wipe, and this, of course, happens way more on the 25men having 6-7 healers to survive compared to the 2-3 of the normal mod. Same goes for DPS'ers not dying etc.etc.