The players who did DISMEMBER-MASSACRE's didn't happen to be giant warriors, did they?

There can be many factors, but here's some:

-Warriors get enchanced damage earlier than assassins(at level 1), which makes them hit harder.

-The said warriors probably were geared for damroll and have higher damroll than you do.

-If they were giants, they have higher strength than you are.

-They might have been more proficient in their weapon skill than you are with yours.

-You might be vulnerable to their attacks(etc. water/fire vs felar)

-They might be resistant to your attacks(etc. physical vs giant)

Most likely, it's either strength or gear that makes the difference, but the other factors come into play as well. However, as an assassin, you have the advantage of them not seeing you while hidden(discounting duergars), so you can strike at them when they have their pants down. And no, you didn't stick a big bullseye on yourself. 2-3 levels of difference doesn't really matter that much.