In my experience, Games Workshop's "heroic" proportions have been some of the more outlandish, followed by Reaper's, particularly in some of the older, hand-sculpted models.
Proportions seem to have become a little more naturalistic with the age of digital sculpting, for whatever reason.
More modern fantasy and sci-fi minis still tend to get bulked up a little, especially around weapons, ankles, and wrists, just to make them a little sturdier for rough tabletop use (those thin areas tend to get flimsy or fragile!), and better-defined - as @D34dly K says, those exagerrated heroic features are easier to sculpt, paint, and see in this scale on the tabletop. (Compare to the sort of HO and 1/72 scale soldiers that "28mm" gaming miniatures descended from, where realistic faces, hands, and weapons are all but invisible on the tabletop, and not very interesting to paint or look at!)
As in all thing gaming-related, though, it's a Your-Mileage-May-Vary proposition. I have and would use realistic scale model stuff alongside the most cartoonish of Games Workshop HeroQuest, Warhammer, and 40K stuff without too much worry, since 28mm and 28mm heroic scaling is so inconsistent anyway, these models are more like elaborate pawns than they are scale models.
If you wanna see something REALLY scary, check out the proportions on these '80s era HeroQuest minis:
Not that I think that's a bad thing: I'm actually rather fond of these miniatures (I didn't paint the ones in the picture, but do I own an unpainted HeroQuest board game - my compliments to whoever did paint them, anyway!) This is an artistic license of style choice, sort of like the proportions on characters in anime and other cartoons, or Barbie dolls, or those strange "chibi" ministures used in some games, and so on. But come on: if you were to see anyone with such proportions in real life, it would certainly be a nightmare from the uncanny valley! :)
Wargames Atlantic's version of "heroic" proportions has nothing on these guys, so from that perspective, they almost look normally proportioned to me, and I really have to compare to reference photos to see the deviations from natural proportions. (The GW oddities of proportion I can see immediate!)