Its not as simple as the math looks like. It all boils down to the amount of idiots you have in your raid.It's easier to have 5 DPS DPSing a Valkyr than 18 DPSing 3, also is easier nuking 2 saurfang beasts between 3-4 than 5 between 18, and the list goes on...
Personally, i feel that 25mans are easier than 10mans, because each person is contributing less, so i feel like im contributing less (and i usually play a tank or healer) If you lose 2 healers on a 25man, the numbers are still fine to beat the encounter, if you lose one healer on a 10man, you might be screwed.
It depends on the fight, it depends on the people you are playing with. Things like soulstones, rebirths, extra raid buffs, extra cooldowns, everything helps in the long term.
The "hard" part of 25man is organizing everybody and making sure they know their class and what they are supposed to do. People are always afraid to answer when you ask "does anybody NOT know this fight"?
Using your example, 5 dps on valks vs 18 dps on valks. It also means more stuns, more snares, and more dps to help on other valks after their main target dies. You kill one valk and help with the other, on 10man you switch back to LK.
On my old guild, my job as offtank was to chain-stun one valk while the whole raid burned the other 2, my valk would get the first aoe stun, then i would charge/hamstring it, then stun again again, and once DR kicked in and the dps were finished with the other valks, mine would just blow up before reaching the corner, where i would charge again (now the DR was finished) and watch it die as 15 people suddenly bursted dps on her.
There is also a reason that people do more dps on 25mans than on 10man (assuming they dont have 400-500ms latency, like me). Just check world of logs.