Hi Mik, do you have the Scene Browser set to be inside the side pane?

It's the setting under Options > General > "Scene browser position: Inside" .

Also re:
> I've noticed that it's file size dependent. Smaller files - side panel is always collapsed, bigger
> files, if "loading file" dialog is appeared, then side panel is always expanded.

Is it possible that it's dependent on having a lot of different named objects and not actually file size?

Thanks,
- Michael