Yep, that was it. 1.89mb for a screeny. Really surprising that they are that much bigger than tech snapshots that have a full tech worth of data hid in there. Then again they are smaller dimension wise. Mystery solved. Thanks you guys.
Well, the data's hidden inside teh least significant bit(s) of the pixel colour coding data, so they are the same size as a regular .png of those dimensions (and image content)... Pretty much.
Glad you got it sorted. It's an idiosyncrasy of the forum that I have listed in my bug reporting guide (but that's too long to expect people to read, even if they know it exists). Be easier if it was just fixed, eh.