Most people that I know that raid don't care about raid size nearly as much as they care about one of three things. A. having fun, b. progression, or c. reward. In most cases, a. and b. are intertwined together. Right now, c. no longer exists because reward is pretty much equal (though the statistical probability of getting a particular item is higher in 25's, that's still not incentive enough to make it equally or more viable of a format option). As of right now, from personal experience of doing both formats in this tier of content, I can tell you that 25-man has stricter number crunches while 10-man requires stricter execution of mechanics. Mechanics are always easier to execute than numbers, therefore by default, 10-mans in some cases are easier. People will take the path of least resistance if reward is equal.
However, if reward is greater, they will take the path of greater reward as long as a. and b. are still connected to c. In other words, if 25-man gives more reward incentive, and people can have fun and progress as easily as they can in 10-man, then people will be more inclined to choose 25-man. Provided the loot is identically styled (in other words, not like the failure of ToC and ICC loot styles that really DID require you to do 25's if you wanted to min/max), then people won't be "forced" to do 25-man because you won't need that gear to progress through 10-man formats. People that want to do 10-man will do 10-man and have gear that isn't at an extreme disadvantage over people who would choose 25-man. Six or so item level is not going to make a difference in any fight in 10-man provided the stats are still identically styled.
As of right now, format is not being selected on what people enjoy. It's being selected on what has a greater effort to reward ratio. Right now, 10-man has a greater effort to reward ratio because the reward is identical and the effort for numbers is a bit lower. To compensate, 25-man should have a higher reward ratio to help balance the scales. That way, people are picking the format they want to play and not the format that has less resistance. I'm not even factoring in the application process that most people just can't be bothered to mess with to join a 25-man guild (which has no relation to the in-game problems of the two formats, but does affect the format differential just as much).
My guild is on the verge of total collapse. I'm sorry, but if Blizzard gave us a helping hand, I would not be incredibly sad to see the online place that I call home crumbling around me. And, for those of you curious, yes. I AM an officer in my guild. I do raid lead in 25's (and I even raid lead in our 10's...we still separate our roles accordingly there, and nobody in the guild knows healing better than I do). I speak from firsthand experience on the issue. If the system gave someone from a 10-man guild just looking to raid a reason to come with us instead of sticking to 10-man, then everything would be solved. Both formats could flourish equally. I think some people seem to suspect that most 25-man raiders want 10-man to be inferior. While some posters here may reflect that type of attitude, those of us with intelligence don't want that. We just want both to be equal options. Right now, 25-man is an inferior option. It needs a boost.