Look once more at the screenshot and see that 800x600 shows a Seeker PU which is not visible in 640x480. I don't play online, so my opinion in this case doesn't really matter, but I find it an advantage, since a player know did it already respawn or not. Such a thing is also useful in new levels, where a high-resolution player will see everything before the other players do, and he will know where should he go to get a PU/carrot/whatever. If someone still doesn't believe that resolution can change player's chances, try to win a duel in 320x240. It's only 5 tiles less, really.

Btw, two and a half of tile more is 125% of a normal range (10 tiles). In 1280x960 it's already 10 tiles more, so 200%. In my opinion it's a cheating program.