"Mex-America" is a weird one (and why is the capital in the U.S.?) too. If Minnesota and Michigan are different nations, then Mexico (which is much more tribally, ethnically, and socioeconomically heterogeneous) cannot reasonably be cast as one bloc. Oaxaca, Mexico DF, the Yucatan, and Juarez are all very different places (just to get started).
Yes, I see now that's described on this page: http://en.wikipedia.org/wiki/Nine_Nations
That makes sense then I guess. I initially questioned lumping Newfoundland and Prince Edward Island in with New England, but from what I know of those areas, it's probably not a bad choice -- they fit with southern NE as well as most of Maine does.
But MexAmerica was not all of Mexico. In Mexico, it was measured by US influence, just as in the US, it was measured by Mexican influence. (IIRC, one measure he gave for the southern border of MexAmerica was where you stopped being able to receive Spanish-language radio stations that were broadcast from the US.)