I've been wondering since release. I don't see why they couldn't have put in 3 sizes of trees, especially because they'd be a simple port from New Leaf.
The four space trees are the traditional tree size from New Leaf? They just take up four squares, kind of like how you can't plant the trees too close in ACNL, but you can still walk under them and stuff.
The four space trees are the traditional tree size from New Leaf? They just take up four squares, kind of like how you can't plant the trees too close in ACNL, but you can still walk under them and stuff.