How can the Hurricane be a heavy fighter when it is smaller and lighter than any of the current medium fighters and most of the light fighters?

The Hurricane punches well outside of its weight class, that's for sure. The slight chassis may read like a light fighter, but in tactical terms that label doesn't fit at all. The Hurricane is not meant to be flown like a light fighter, if by light fighter you're going to imply 'dogfighter'. Pitting a Hurricane against a Gladius or Avenger is certainly feasible, but it is an unnecessary risk - 'overwhelming force' is the entire focus of the ship, and it is meant to be used against heavily fortified ships and outposts, especially the kind that an average fighter's loadout might struggle to hurt without risky prolonged fire and the exposure that comes with it. In-game and historically, it's also worth remembering that the Hurricane's design originally hails from another time, and for its time it was certainly pretty heavy.