Well, no-one says those are adult dragons ...
WizKids does themselves. Look at the
descriptions for the various Attack Wing products. The green dragon (which I believe is technically Large) is identified as Claugiyliamatar, a
famous green dragon from the Forgotten Realms who featured in the first D&D Epics adventure,
Corruption in Kryptgarden. She is an ancient dragon, which in D&D 5e terms would make her Gargantuan, not Large.
Likewise, the silver dragon from Wave 2 is described as being "one of the most ancient of her kind", which again should put her at Gargantuan size.
Now, you could argue that the Attack Wing minis are part of their own game and therefore don't need to be sized according to the D&D 5e rules. That's fine. However, WizKids is then taking those minis and putting them in the "Icons of the Realms" series, which, as far as I can tell, *is* for use with the RPG. As an RPG player, I find it annoying that the official 5e minis aren't all the right size according to the 5e rules.
But really, the elementals? They can be of ANY size, it depends on their HD, at least it did back in the old days.
It looks like you don't actually play 5e. This is no longer the case. The elementals presented in the MM are all Large, and the minis have been modelled to look just like the ones in the book, so I don't think it's unreasonable to expect that they all be the right size, instead of only some of them.