It is honestly just a mixture of both. There are casual/semi-hardcore/hardcore alike in both 10 and 25mans.
Obviously 10mans would be easier to put together for a guild than a 25m so yes the number would go down b/c there are flat out more 10man guilds than 25m guilds.
The 10man obviously have their own challenges with alot more dependance on personal responsibility and if you lose a person it is very intense to get them back up or it could mess the whole attempt up whereas 25m can lose a few people and it is not so bad. In the same token if you are able to form a 25m than you should have an advantage to use those number of player to ur advantage with more CDs/utility/etc.