Tank to healer ratio is far better in a 25, guess which one will be receiving more consistent heals. One dps dying in a 10 is a 20% reduction in raid dps. You will get geared up faster in 25 man because you have a much greater chance of seeing the item you want to drop. 25 man gets 90vp (why not 70vp like 10m this I dunno).
Blizzard way of tuning 10m by taking 25m and dividing hp by 3 was a terrible way to tune the fights, blatantly obvious with the huge amount of 10m nerfs they had to come up with. I bet also a 10m raid dps is not 3x less than a 25m raid dps, probably more than 3, so that dividing the 25m hp by 3 isnt even correct. One huge oversight by Blizz