At that point, like Spiritus pointed out above, the risk has more to do with unreliable players. But an problem of unreliable players, or a D/C, could go both ways. A 25 man raid may have more people, and thus more opportunities for there to be an unreliable player, but the overall impact of one bad player will be less in a 25 than in a 10. So in that case, a 10 might be riskier.
Then again, this is really all just speculation.