I was thinking this before scrolling down to see your post. It's consistent with the rest of the game as you can upgrade the Farmhouse and the Sheds so at least one more size upgrade to the Greenhouse would fit the game like a glove.
Yes I realize that Ginger Island is essentially a giant Greenhouse but it's really a separate Greenhouse-like entity hidden behind a big 10 Dragon Tooth smile and more than a few Golden Walnut puzzles. And to me it feels very disconnected from the rest of Pelican Town even when you get your Return Sceptre and Island Obelisk running. It's too easy to ignore your spouse/family/real Farm and hang there for a bit. OK maybe my feelings are colored by Haley dealing with a new baby delivery the very first night I spent on Ginger but that does make it difficult to RPG the game when you get stuck in that situation.
Nah, an expanded Greenhouse is totally the way to go and would foster more and earlier interest in expanding the family Farm. Plus I want to make a nice Tea Room West for Caroline to visit but not sacrifice too much floor space.