Hi Chris, I'm not sure why I'm leaping in here, as I haven't really got a clue and I haven't got time right now to check out my theory on one of my pics. However, I think maybe your edge measurements might be just in pixels not ppi and the 300ppi is 300 pixels per square inch.
In which case, your picture is 1800 x 1800 pixels = 3,240,000 pixels; at a print resolution of 300ppi, that gives a print area of 10,800 square inches (eg a print something like 108 x 100 inches).
Does that make any sense in what you're getting, or am I talking complete poppycock??!