10-mans were tuned much lower than 25s on live during Wrath, even accounting for the difference in gear level between the average 10 and 25 progression raider.
This isn't about which was harder - which for my money is plainly 25, as the difficulty of progression raiding is in getting everyone to perform up to the necessary level, a task which is necessarily more demanding with 25 people in the same encounter. Likewise, voidzone mechanics like Defile are plainly harder to deal with in 25. But there is no argument that 10s were tuned as hard or harder than 25-man raids in Wrath - they simply were not.
EDIT:
10-man raids were not an attempt at tuning parity with 25s until the big revamp in Cataclysm, where 10 and 25 also rewarded the same gear. Even then, only T11 really had a majority of fights where you'd argue that 10H was more tightly tuned than 25H, and that was because they were ****ing it up since it was the first tier they were doing this in. Fights that were notably harder on 10H (Garrosh Hellscream in MoP comes to mind) were few and far between.