1) You have to FIND 25 people to raid 25 vs 10. How many pugs excactly raid 25s in your server?
2) You have to find 2,5 times more people to maintain a 25 due to the natural afks departures etc etc.
3) You have to spent more time to form a group of 25 people vs 10. Even if you have them all showing up, you need 2,5 time larger bench than 10 and that means more time to decide who stays out and then deal with their dissapointement for staying out.
4) You have to get em inside the raid. Especially now that "have group will travel" is gone.
5) You need more time to go through tactics and assign roles.
6) You need to go through the tactics every time almost since it is highly unlikely to have the excact same people that you raided with the previous time and the new ones have to know how the rest work on this.
7) You need more time to distribute the loot. 6 items for 25 people as possible recipient while in 10 man its more like "oh agility leather give it to the rogue".
8) You need more time to recover after wipes
9) You need more waiting time from people to come back from unexpected afk/random DC
10) You need more time to actually kill the boss in combat since most of the fights last longer in 25 vs 10.
Basically you know all the above. But making naive comments to descredit people, even if those people happen to be Blizzard's Lead Encounter Designers is a very popular tactic in forums after all...