There is no higher skill needed for the chance of failure to increase.
No, you don't. You need the same skill as in 10 man. What you don't get is the same chance at a reward.Yes, there are more people in a 25 man, so given equal player skill you will wipe more times in 25s to kill the boss (more people who can screw up and cause a wipe). This is why you need higher personal skill and performance in 25s for everyone in the raid.
I gave the example earlier of lifting and lowering your knee - it's just as easy to do on your own as in a group of 25 people. However, if your reward is based on all 25 of you doing it without messing it up at the same time, then your chance of being rewarded goes down. The actual difficulty of the task remains the same, however.
To say that 25 man is more difficult you have to point to extra tasks that need to be performed while playing the game. The argument is solid if you look at the raid leader, and maybe the healers have to alter their playstyle a tad. For the DPS and tanks, on the other hand - same deal as 10 man. So out of 25 people maybe 4 or 5 have something different to do then if they were in a 10. Does this justify all 25 getting more reward? Can't see why it does, really.