Kdenlive Render makes enormous files 6 times larger than the one used originally.

It does not matter what format it generates 3.2GB from a 500 MB file

What am I doing wrong?

Thanks

You must adjust the quality slider in the render dialog, by default it is set to a very high quality
After processing, the editing engine must necessarily re-encode the video, this re-encoding is not related to the original encoding.
Also check that the project resolution is not bigger than source material