[quote name='spacey' timestamp='1409580718' post='2541097']
Think speaker inches.
1x15 is 15 inches of paper moving air.
4x10 is 40 inches moving air.
[/quote]
No, it's volume of air displaced that counts. The area of a 15" speaker is 225sq" [1] and 4x10" total 400sq". I haven't studied Xmax in great detail but I suspect that a 15" speaker would tend to have a bigger Xmax than a 10" speaker, so that brings them closer together in terms of air volume displaced.
[1] Well, it's bigger than that as it's a cone. However, imagine a spherical cow...