a players hitbox stays the same, so most of the time, even if you miss just by a bit you will still hit. the hitbox does not change at all, however having no head will make the person slightly harder to see... slightly....
i tried using arachna's head on someones body, and you can still see it, but its tiny. perhaps this is what that headless person had
sorta unrelated:i used arachna's head on sergeis body, using gauntlet only and my name was ^gauntlet^pure, beat quite a bit of ppl too
(wish i could find my screenshots of the scoreboard at the end of the game)