Case in point: Hodir Hard. In 25-man he has 32.5m HP and the average raid might have, at most, 21 DPS for this encounter. (32.5m / 21) / 180 seconds = 8597.88 DPS each. On 10-man he has 8.4m HP and you'd likely have 7 DPS. (8.4m / 7) / 180 seconds = 6666.66r DPS each. In terms of what's expected of your raiders, it's clear that the 25-man version represents a significant increase in DPS, even if not specifically skill. The difference in gear you get from 25-man against its 10-man equivalent does not make up for such a wide difference in the output required of your raid, so it would typically be something you completed later than the 10-man version. Thorim Normal is another fantastic example of an encounter being far harder on 25-man than 10-man.