"28mm", "heroic 28mm", "30mm", "32mm" is all b******t. It's not just the heights of the miniatures (which as anyone who plays Cygnar in Warmachine will tell you, varies wildly in the same range), but the bulk and the proportions. Then there's the question of are they measuring from the sole of the foot to the top of the head, or to the eye (that's always good for an argument on wargames forums). Knight MOdels advertise their smaller Star Wars models as "30mm", yet Han Solo is 32mm to the eyes, or 35mm to the top of his head. Harrison Ford isn't renowned for being a giant, so Knight Models are just being misleading.
Infinity, by the way, describes itself as "28mm" (and GW haven't bothered to describe their models in terms of a "standard size" in years*) and even then, the newer models are noticeably different to the first few.
this guy is massive compared to
these guys even though he's allegedly the same troop type. About the best you can do - in the absence of a side-by-side photo - is compare them based on base sizes, and hope the perspectives don't throw things out.
Frankly, it's only a problem if the models have details that should be the same size but aren't - for instance if I'm painting WW2 Germans, it's OK if one guy is 27mm tall and another is 29 or 30, but they should still have the same sized K98 rifle)
*since I had her handy,
this lass (on foot) is 29.5mm to the eyes and 32mm to the top of her head. I chose her since she's the only GW human Warhammer model I have.