Why does some frozen food puff up when defrosting?
That's just the air in the package expanding, as it goes from freezer temperature to room temperature.

It sounds like you're thinking of the fact that water expands when it freezes (and shrinks when it melts) but that's not what's happening here. The air warms up long before the ice starts melting.

Some freezer packaging has small holes in it (or not so small holes, if you're unlucky), which prevents this from happening.