If you haven’t been to the Alamo, yes that Alamo, in San Antonio, it’s way, way smaller than you probably think it is. You might be shocked at how really small it is.
Not a great surprising fact but I was thinking tonight about some of the historical sites I’ve visited and that came to mind. I might have a better one later.
Edit: I reread your question and may have missed the point there. Though the size of the Alamo can be objectively measured, the issue of it’s actual size relative to how big any one person might imagine to be, well, that’s a little soft and outside the parameters of your question, I think. Sorry.