Hi DnDChick, I really like your system. We have been trying to use the C&T 2E crit charts in our games, but I think I prefer your system. One thing I liked about the C&T charts though that you may want to consider however, is the severity being affected by weapon size. While I agree with you argument that the size determines damage and hence affects the save DC, it is just as likely to roll a 1 for damage on a great axe as it is on a dagger, and hence each is just as the same odds of severing a limb.
Maybe you could add a modifier to the severity roll based upon weapon size vs opponent size, say +2/-2 per size difference. So the halfling with the dagger is fighting an ogre with a great axe and both subsequently score criticals on each other. The halflings dagger is tiny and the ogre is large, a size difference range of 3 smaller, so he rolls d20 -6 to determine severity. Conversely the ogres huge great axe is 3 sizes larger than the halfling so he rolls d20+6 for severity.
The halfling has a slightly greater chance of causing a critical (19-20) but is likely to only be minor, where as the ogre is likely going to severe a limb it he makes a solid connection with his great axe.