It all depends on how you define "parry". The badass, princess bride, errol flynn style, feet firmly planted on the ground and dashing the other guy's sword to the ground thing isn't gonna work here, of course. If you just stand still and hold up your foil like a "karate" block against a bigger sword or axe, you're goin' down hard

. And you won't parry anything with the weak side (the tip), be it a battle axe or another foil or a wet rolled up newspaper (i joke, i joke). That's just simple physics, like you say DD.
But the strong side of a fencing foil (the first 1/3 of the blade from the hilt) is surprisingly sturdy. Engage the other weapon properly, and I'd be willing to bet that a properly trained fencer with a properly made sword could successfully parry even a full on two handed swing from a claymore. Contact with your midpoint at their weak side (i forget all the fancy shmancy french words for this stuff), then slide up and jam your hilt up against the other's blade or axe haft while they're swinging away, and you can get just enough control over them, while at the same time you're using your footwork to either turn the corner or get inside (totally illegal in fencing, but whatever

). It's not at all easy, but possible for a good swordsman i'd say. You don't have to stop the other weapon, just redirect it a few inches one way or the other so you can gain the advantage. That's all i meant by the martial arts comparison. In judo, or kung-fu, or whatever, it's not about being the bigger dude, it's about controlling your positioning and mitigating the other's strength by proper engagement. I know it's not exactly the same, but i've rolled with dudes literally twice my weight in wing-chun, and if you do it right, you can push them around as easily as they can you. The little guy just has a much smaller margin of error of course.
DigDog wrote:If anything, wielding a rapier or foil should give a bonus to dodging.
And that
Sorry to ramble, none of this is relevant to game mechanics, but i find fencing and martial arts really fascinating. And i'm pulling an all nighter. . . What the hell was my point? oh yeah. . .
tl;dr: a rapier is as good as the person holding it, no more, no less.
"Bein' a minotaur is a lot like bein' a regular human except moo" - J. Rowland