I am not sure about international transfers Terminal One in YYZ, but one would assume it's the same as in Terminal Three. When one transfers to an international (as opposed to a transborder) flight you do not have pick up your bags and recheck them. Just mosey on over to your new gate and meet your checked bags in MEX! Of course, for the return trip, this will not apply, as you will have to claim your bags to go through customs in YYZ before rechecking them for your domestic portion.