I think the developers chose the map size for performance reasons. Yes, the GameCube map may appear larger, but that's because everything was split into small acres, low texture detail, little to no clutter around, and, probably the biggest of all, the world was static (trees don't move). In New Horizons, every leaf/fruit moves independently. And that's in addition to the gorgeous textures and 3D models, clutter, and so on. AND of course, all of the furniture we'd have laid out around our town at varying levels. So, to me, that's console quality.
With that said, I do hope we one day get a world that feels as large and immersive as the Mario Kart 8 track or the anime. I personally think that'll be the direction the next game takes. With that said, New Horizons is still making huge strides.
Edit: Completely forgot to mention furniture outside, lol.