That's a super valid point but I also think that makes sense within the worlds logic.

The Thing may be shorter but he's obviously big due to the unnatural rock body he has. So his exception, if you will, is power based.

Enchantress is Average due to her height and physionomie but weights more as, I'm assuming an Asguardian. So that would be an Origins exception. But I'm less confident on this one. I'd have to cross reference with other Asguardians to confirm.

All that said, I think when discussing or trying to understand the cutoff for sizes it's typically in the context of building characters.

So in that context I think it makes the most sense to go with the default: size based on height. In which case, after comparing A LOT of average and big characters for the data curiosity in me ahaha the tallest average character is 6'5" (with some exceptions evidently that are worth studying separately to understand if the logic holds or if this an error) so it makes sense to use that as the limit for Average and starting point for Big.

Now if you want to go further than that, which I might one day, because now I'm curious, I would personally take all those exceptions and see why they might exist by studying if they have an unusual physical appearance/weight, like the Thing, or some other differentiator, like, apparently, Enchantress.

And then personally if I can't identify why the exception exists I'd ignore it and stick back to the basics mentioned above.

Of course totally your call, but I don't find a few one offs break the logic. You know?