Also, the 'vi' on most unix/unix-like systems aside from Linux these days is nvi, not vim. which is about 200K, not 2G.
And you completely miss the point of mg:
> mg is intended to be a small, fast, and portable editor for people who can't (or don't want to) run emacs for one reason or another, or are not familiar with the vi(1) editor. It is compatible with emacs because there shouldn't be any reason to learn more editor types than emacs or vi(1).
> mg is just a text editor with Emacs-like bindings which is not worth much to me.
It is of worth to me when I need to edit files on an other machine than my own, I've got the basic navigation and edition bindings embedded in my nervous system (even more so as they're also the edition shortcuts of standard Cocoa controls)
> Also, the 'vi' on most unix/unix-like systems aside from Linux these days is nvi, not vim. which is about 200K, not 2G.
I said 2M not 2G. And OSX uses vim as well, so most active VIs are probably vim not nvi. But even assuming it's nvi you have, that merely makes it as small as mg, still no justification to not have mg.
 I'm not sure any non-BSD unix-like uses nvi actually, MINIX has switched to elvis and OpenSolaris looks like it uses vim.