Where are you seeing that your document is only 600MB ? If it is the size at the bottom left of the window then that is the size in memory, but it only counts smart objects as if they were a single pixel layer at document size - even if the smart object contains multiple layers at different pixel dimensions and possibly other nested smart objects. It also excludes metadata.
If it is anything else then please show further what the 600MB is based on.
The file size in Windows File Explorer is at 652mb atm: Also don't have any smart objects here, all raster layers, and nothing is more than a few pixels outside the composition. So I have no clue why it's jumped up to 2gb all of a sudden.