Brad, about the skinny top variety on a fat rootstock: I've seen this once in person. It wasn't avocado; don't remember what fruit, much less the varieties of rootstock & scion. But the thing ended up dying.
I've also seen the opposite: swelling scion on top of normal-size rootstock. That one I have in a pot myself: Persian black mulberry on top of Pakistan black. That combination consistently behaves this way. I've seen it in a friend's full-grown mulberry tree also, with the same arrangement: Persian on top swelling, Pakistan on bottom normal. I'd guess that there's some kind of "incompatibility" (for lack of a better word) when grafting certain combinations -- even though both parts of the graft are the same kind of fruit (apple on apple). In some cases it looks like the sap flows up better than it flows down; in other cases, vice-versa. I've never heard any biological specifics of why or how. I'm not sure anyone has studied it enough to know. If they have, I'd like to know.
