PokerTracker

Is this normal? After converting a PT3 DB, it's around 10x bigger than the corresponding PT2 DB file. I have both storing HHs.

Yes, the PT3 database saves a LOT more data than the PT2 database. As a result, it is much larger.

I wasn't expecting a 4M hand DB to be over 33GB when it was like 2.5GB with PT2. Is there, could there, be any way to streamline this? Can I delete variables or columns from the SQL structure to save size without totally boning everything?

No, I doubt it, unless you REALLY understand the structure of the complex PT3 schema.
You will almost certainly break something.

Make sure you vacuum your database, as that may dramatically decrease the size.