I think your logic is very confused here. We're talking about a theoretical new 7" product here. Your numbers should be comparing that 7" product against the current 10" and 3" products to find out which one it's closer to. So...
10-7=3
7-3=4
Therefore, it would be closer to being a small iPad than a large iPod Touch.
Your numbers compared the current iPad to the iPod Touch, which is really irrelevant. Actually, this entire line of thinking is irrelevant, because size alone isn't the determining factor. The usability of the product is. If it's 7" but functions and has the capabilities of the iPad, then it's an iPad mini.
You're measuring by length. You should be measuring by area. Maintaining a relatively constant aspect ratio.
iPad 10 in x 7 in = 70 sq in
Mini iPad 7 in x 5 in = 35 sq in
4" i phone 4 in x 3 in = 12 sq in
Areas get larger by a larger ratio as they grow. The iPad mini would be closer to a larger iPod Touch.