Aye, GC was indeed 16mb. I think New Leaf's size may be huge due to the 3D rendering?
Nah, 3D rendering has nothing to do with file size. The DS had very little in the way of memory, so they kept most of the low-res textures and models from the GCN version.
While wild world doesn't LOOK much better, it actually has a lot of improved models and the textures are probably 8x as large (and more of them) which is why it's so large compared to those 2 (no need to waste time compressing the textures either, streaming from the disc would be more than enough)
3DS has more RAM than the Wii does (not sure how much available to developers, but even then it'd be on par with wii) so they can afford to use high res textures and models, plus they seem to have added a ton more content. That's why it's so large by comparison.