It depends on the situation, and obviously balance is what is being aimed for by the designers. However, I've got to agree with what Ensis said earlier on: If everyone controlling the characters were of average skill, I think the heavy would have and advantage. Bigger weapons, more health. The game looks fairly fast paced to begin with, and lights don't have much health. Considering heavies can carry heavy machine guns, unless the light is intentionally stalking the heavy and manages to get the drop on them, the light wouldn't stand a chance. Image a light or heavy rounding a corner, and coming face to face with their opposite. Unless the light has somewhere to run, all the heavy has to do is keep the light in their cross-hairs for a little while and keep firing, and the light is toast. Same goes for medium versus heavy, except the medium would stand more of a chance, and last a bit longer.
Now, just to be clear, I'm not saying heavy is the "best" body-type. Simply that the heavy is more prone to survival than the others. In combination with their heavy weapons this makes them easier to use so, again assuming everyone is of average skill, it seems the heavy has a definite leg up.
Of course, the "rounding the corner" scenario isn't the
only scenario that would happen...but it is a common one. If a light can get
behind a heavy it's a whole different story, for instance.

This could be done either by the light's more agile SMART usage, or a light Operative's disguise ability, although that brings class into it. Point is, a heavy wouldn't win all the matches, not by any means, but on the whole they seem to have a bit of an advantage on average.
That said, heavies are kind of boring. I'll probably be playing mostly light or medium, although I can't say for sure until the game comes out. Agility just seems more fun than brute force, though.