The physical dimensions of the wheels are one of MANY variables. The -9's have newer more modern wheels which flow more and more efficiently without needing to be bigger or lighter.
To the previous posts about the -7 turbos, I'll say it again, -9's AREN'T any laggier than -7's so there is NO advantage of -7's! If u only want to make a certain amount of power, just run -9's on low boost!