Just take something as simple as distributing loot. In 10man you need and greed, pass to your fellow raiders and use common sense (in 99% of the 10man raid teams this happens). In 25 man you need to figure out a complex loot system to keep everyone happy and give everyone the feeling they are getting properly rewarded. Which brings even more problems with people trying to game this loot system and pissing off others in the process. Its just far more complex, and 'more time' is certainly not the only thing it takes to manage.