Well, I think physical size, and the "size" shown in the screenshot are two different things. I think that "Size" is a measure of computational complexity - how many polygons/parts/etc have been placed in the cell (hmm, with some overhead included to take into account the NPCs, and raider attacks).
So, even if the plot was 5 acres..... two dozen simple buildings spread across the thing, and one really complex building sitting alone in the center, might still show the same "size".