There is no way to get a different result - except you have been confused by the step by step processing.
1. hatch:
This step is to confirm that the contour is closed, correct? Nothing else is behind this approach. Hatch possible - great - forget it.
2. polyline:
Then you need for some reason everything converted to polylines. QCAD has a bunch of tools to do that. You used OG (I don't told you that!) but it wasn't working as expected. So, now you are investing a lot of energy to "fix" OG instead of trying one of the other "Create Polyline .." tools.
Please look at every tool set like the offered circle tools. There are at least 10 circle tools, every tool is for a different situation but at the end of the day each of them will simply create a circle!
Back to the polyline task ....
If you select all, launch the OC (!) tool, QCAD will convert everything into polylines although the given shape is not perfect, no need to play around with tolerances, task will be completed successfully.
3. svg
export the polyline drawing to svg. There should be no need to scale anything because that is one of the benefits of a svg. It's a vector graphic which can be scaled in any application to what ever size is needed.
Note:
Your drawing is imperfect. Maybe because a lazy untrained drafter created it or because an other software or converter messed it up. Howsoever - QCAD is not built to fix a third party imperfectionism. Yes, luckily for us it is able to fix a lot of those things but that is not the idea behind a CAD software.
Closing argument:
Analyze always the given task. In a case like this one you have two ways to address the issue. Fix the drawing to make it perfect or deal with unexpected QCAD tool behavior because the tool has to handle uncommon situation. If you have to deal with unexpected behavior think about to attack the issue from a different angle ....