Scale is a very tricky issue once you hit that size. It's not even as simple as "if it's the right scale it's all good".
Some models and terrain actually look odd at the correct scale. This is because of things like bases, unrealistic proportions and dramatic posing....it's especially noticeable on buildings where doors and windows sometimes look funny, but you can see it on vehicles too.
Added to that is that some things, if accurately scaled, are just too big for wargaming. Not even things like AT-ATs....Justin take a look around at the buildings and houses near you. Now look at wanes table buildings....in general they are tiny compared to real buildings, because otherwise the table looks odd and plays badly. This is due to ground scale very rarely matching model scale.
Most wargaming stuff takes some degree of liberty with scale so that it "feels right" for these reasons (though I should add at historical gaming tends to be more accurate, as sizes are less insane than fiction).
In regards to an AT-AT specifically, it doesn't necessarily need to be accurately scaled, but it DOES need to look right compared to over models. Accurate scale would actually be enormous (the aforementioned Revell model is roughly correct) at around 35-40cm. That's more than a third of the table width, and leaves very little room for tactical play or movement.
It also absolutely dominates the table aesthetically, which isn't always a good thing. It draws all focus to it, and makes the game as a whole look quite secondary (which might be the case in a gameplay sense too).
So scale....it's tricky. I don't think we'll ever see an accurate scaled AT-AT, and oddly I don't think it'd look amazing if we did. I also don't think it'd play great.
I will still however be getting a Revell one for display purposes....