The entire "Line items" subtable is indented: its internal horizontal lines do not stretch all the way to the vertical line at the beginning of "Line items". However, the horizontal lines between the line items for different orders do. This subtly highlights where one order ends and the next begins; I think its quite clever. Also, the "Qty" could be made thinner if you wanted. But in the screenshot there is still space available to the right, so its not vital. (Also, there might be a three-digit Qty further down the table, which would make this the right width.)
I don't find the "Folders" side bar that vital and I'm sure it could be improved. When saying that I think the interface strikes a good balance, I'm focusing on the main view. However, I think that there could be extra root-level items if you add further data sources, and I also think that you can make the sidebar thinner, at which point you will only see Cat, Cust, etc. For a main screenshot having the full names visible does seem cleaner, though. (I'm not entirely sure that all of this is correct. I cannot test it--unfortunately, while I think that Ultorg is very smartly designed in many places, its license cost is not justifiable for my limited use cases.)
There is already red bar above, it doesn't explain the waste, it's too small. But also it doesn't need to point to the left of the same spot, you can move it to the right to grow left - will be even easier to compare the absolut sizes.
> as nested tables are slightly indented
For nothing. Compare indentation of Product and Qty - both indented, so the indentation of Product is useless in signaling "nested". Headers is the only "reliable" place to read that info
> does not have an arrow
which doesn't need to be so big, you anyway should be able collapse/expand by clicking on the whole big field
> Therefore it seems overly indented
No, it is overly indented because the indent is too wide, especially for the first level - it shouldn't even exist, it's pointless separating the only Root from it's children with the same space as further levels where you can get confused, it's like the common fail of Jsons (which must have root {}) of wasting indentation space for each line
> entire side bar can be hidden.
Which is also no solution to this waste. Instead it should have a "condensed view" where you can still read the first couple of letters of Cat/Cust/Empl etc. If you want to be binary - either the tree is useful and should be visible at the limit and only disappera when you really need the last 1-2 columns it displaces, or it's not, and shouldn't be shown at all. That's useful efficiency