My understand is this is largely because the land mass was pressed down by kilometers of ice. With the ice gone we can expect it to spring up very quickly same way Scandinavia did and is to some extent still doing today.

I have no idea how much can be expected to come up but my very quick and inaccurate estimation would be something like 1/8th to 1/5th of the ice thickness pressing it down.

If the land mass springs up, does somewhere (everywhere?) else on the planet drift down to make up for the change in volume?

Indeed. It's one of the Atlantis theories. Idea is that as North American ice disappeared the land lifted up, Atlantis was on the other end of that plate and went down into the sea respectively.

Randall Carlson has some interesting theories.

Keep in mind that "spring up very quickly" to a geologist is on the order of inches per year.

There are several places currently where glaciers have melted and are experience uplifts on the order of inches a year.