If they're already balancing the "numbers" for two raids, its easy enough to extrapolate decent numbers for the remaining amounts, and imbalances are more easily identified.
Mechanics could be added at varied group sizes as well.
ie... 10man has 1 add every 30 seconds, 20 man has 2 adds, 30 man has 3 adds.
15 man adds a mind control effect, 25man has 2 mind controls.
I doubt many guilds would actually aim for 40man raiding, but for those few that do, it would be a bragging point. (and 40man farm raids would be mad fun too).