Why does some frozen food puff up when defrosting? I know freezing is supposed to make foods expand but it seems that two pre-packaged sealed food items that I have in my freezer seem a bit puffy (chicken curry and chicken dumplings with cabbage) after taking them out of the freezer. 

I don't think they were that way when I bought them but not sure. My freezer seems to working well (although my fridge is a bit variable) and foods always seem well-frozen. And I only left them out less than half an hour.

Can sealed food packages expand when they defrost rather than the other way around? Thanks!
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.