Doesn't Australia famously have no emissions standards for cars (until recently/soon?)

Which would suggest it's a wider trend and not just Obama at fault.

I'd suggest people getting older and bigger (both height and width) has had an impact. It's just easier to get in taller cars for some people, like my in laws.

Relative wealth (there used to be a lot more tiny cars for basic economic reasons).

And a safety arms race caused by other cars being bigger for the above reasons.