For most heroic bosses, it seems that 10-player is currently harder than 25-player. Interestingly, the two bosses where it is clearly much harder on 25-player both have chain lightning abilities.
Which brings us to the obvious question of what abilities actually make fights harder for one size of raid than the other. Here is a quick compilation of things that the developers should be keeping in mind when balancing the next round of raid encounters for 10 and 25 players.
When there are more people, there is less room for each person to stand. This is pretty obvious, but it doesn't seem to translate into ability balancing. If a chain lightning jumps 8 yards on both 10 and 25 player mode then it is pretty easy to stand in appropriate positions to avoid jumps on 10 and quite difficult on 25. Since chain lightning often does more damage with each jump, 25-player makes it is very harsh mechanic.
Solution: Vary the jump distance for 10 and 25 player. With a shorter jump distance on 25 player or a longer distance on 10 the mechanic won't favour one size or the other.
A typical 10 player raid might bring two melee characters, while a typical 25 player raid may have five. Many melee hate mechanics much more difficulty with more melee characters. Think of Kel'Thuzad's Frost Blast. In 10-player, there was no need for more than one person to get hit by it at a time. In 25-player, brining more than three melee meant it was always possible.
Solution: Significantly reassess melee hate - this needs to be done anywway.
Limited Access Abilities
This is a wide range of different abilities that are only available to certain classes and specs. I'm not talking about the fact that it's harder to get all the buffs you need when player with 10 people than it is with 25 people. I'm talking about interrupts, damage reduction cooldowns, Leap of Faith, AoE slows and other abilities that make boss fights much easier. In a 10-player raid, you may have no AoE slow, making the fester blood part of Cho'gall extraordinarily difficult. On 25-player you should have four or five interrupters available to stop a mob that needs to be interrupted regulary, in 10 you might have just one or two. The less specs that can use an ability, and the more uesfull the ability, the more likely it is than 25's have the edge.
Solution: Consider which abilities have a strong influence on the ease of the encounter and whether those are widely available or open only to a few specs. Especially do this with Hand of Protection.
In 25 player you have a pretty easy option for a third tank, in 10-player a third tank is not very viable for the majority of fights. Putting in a large number of adds to be tanked dramatically shifts the difficulty from 25-player to 10-player.
Solution: Vary the number of adds, not just the add health, between the difficulties.
There are plenty of other asymmetric abilities between 10 and 25 player. Most 10-player encounters still feel like afterthoughts based almost precisely after the 25 player version but with lower numbers. Blizzard still has a way to go in terms of understanding 10 player raiding and the same mechanics feel so different when there are different numbers of players up against them.