I'm not an expert in how this is usually done, but logically I think this would work.
I used the ruler tool in photoshop to determine that the trackpad is 40.12% of the width of the macbook (using the photo that shows it from an overhead view.
The MacBook according to the tech specs page is 11.04 inches wide. That makes the trackpad 4.43 inches wide.
Doing the same thing with the 13" MacBook Pro Retina, I got that the trackpad is 33.2% of the width of the computer.
Doing the same math that makes the trackpad on the macbook pro retina 4.10 inches wide. If the official specs for that have been posted somewhere, you can use that to figure out how close I am with these measurements, but I couldn't find them anywhere.
So I'm guessing that the trackpad in the MacBook is slightly wider than the one in the MacBook Pro.