However, it's not clear there's anything that small states can do about this (other than make sure that this doesn't get passed in states making up a majority of EC votes).

The states that would arguably be stupid to go for this are actually the purple ones rather than the large or small ones, because they are the ones that are currently the focus of presidential campaigning but they would no longer be if there was a mechanism for making the EC reflect popular vote. That's perhaps why Colorado may remove itself (see https://fivethirtyeight.com/features/the-movement-to-skip-th...)