Because they feed on it! Just kidding — actually, fear just makes spiders look bigger, as scientists determined when they had a bunch of volunteers hang out with tarantulas and then guess their size. People who were afraid of spiders estimated them as bigger than did those who were just fine with the eight-legged guys. And overestimating their size might have made the subjects more afraid, leading to a terrible spiral of panic and doom that leads directly into the maw of Shelob. Time to despair.
It doesn't matter what size they are. As long as they are still able to invade my orifices on the sly and lay babies who will then eat their way to the surface via my skin I do not want any thing creep crawly or winged and creep crawly anywhere near me. And by near I mean within a 50km radius of my person.
Also, tarantulas in cages for some reason are cute.
ETA: I really do believe this it is not an exaggeration of my fears (though my fears are an exaggeration). It's all I can see in my minds eye when an insect is near.