I think its a combination, but ultimately I agree the scale is larger. Its exaggerated by two things in particular, larger bases (both wide AND tall) but also the much more kinetic poses. Look at the skeletons. A lot of the older ones kind of hunch forward as if marching. Newer ones are both standing taller, not as hunched, but usually broader. Makes them seem much bigger.
BUT on top of that a lot of the models ARE just bigger. Some of the human characters can be nearly space marine size at times, although thats MOSTLY particular characters like the Cursed City ones that are designed to stand out. I have a number of AoS elves and they're generally the same size as the old ones, but with some more realistic proportions. Dwarves are the same. When I match my slayers and fyreslayers up, the models are ROUGHLY the same size, but the fyreslayers look bigger because of the much more giant crests, dynamic/broader poses, and the little bit of increased scale.