Interesting "Automatic Quality" export results

Hi everyone,
I normally use DR to create online courses (Full HD) but today it's a little different, I am using for old family memories, videos dating back to my childhood!
These were likely filmed in Super8, later converted to VHS. I sent the VHS to a shop who digitalised them for me, and they produced 720x576 mp4 files (H264).
Each file is a 60+ minutes of videos, which I want to slice into smaller clips for easier family sharing.

That's when I got interesting results when fiddling with my render settings, having selected custom export, mp4, H265, for a 4 minutes clip:
=> With 10,000 Kb/s (my go-to setting for eLearning videos), the exported file weighs 308 MB.
=> With the "Automatic" option, I get a 45 MB file.

For memory, the DR Manual doesn't say what exactly "Automatic" does, but I had read in a few places that it would create huge files and I was better off capping the bitrate with 10,000kb/s or another value of my choice.
In this scenario though (old low quality footage), the "Automatic" footage is much more file size efficient.

Is this expected? Would you expect any trade-off in going with the "automatic" setting and enjoying much lighter files?
Of course I scratched my eyes comparing the 2 different exports and I am struggling to see much differences. Yet these videos are quite emotional for me, and I don't want to compromise quality, but I can't go and do a side-by-side comparison of the hours of footage we've got...

Please help me make up my mind about whether I am sacrificing quality with "automatic" quality, in this scenario

cheers,
Nicolas

1) Cue Marc and his "h264 is bad for post" post

2) Which "Automatic" setting did you use? Best, least or somewhere in between?

3) "With 10,000 Kb/s (my go-to setting for eLearning videos), the exported file weighs 308 MB."
Is that Constant bitrate or Variable?
I would use the Constant QP of 15, 15 and 15 (cue Jim and his screenshot) to make sure that the quality is visually identical to the source.

4) I'm not saying that you shouldn't trust your eyes, but I would advocate a mathematical comparison instead: https://github.com/fifonik/FFMetrics
I would render the clip with FFv1 RGB 16bit, use that as the baseline and compare the other two encodes to the FFv1 to see how different they actually are.

2) Which "Automatic" setting did you use? Best, least or somewhere in between?

3) "With 10,000 Kb/s (my go-to setting for eLearning videos), the exported file weighs 308 MB."
Is that Constant bitrate or Variable?
I would use the Constant QP of 15, 15 and 15 (cue Jim and his screenshot) to make sure that the quality is visually identical to the source.

Thanks for your replies
I use DR 19 on Mac OS and haven't seen any of these options offered to me at export. Which is probably fortunate, as I have a very limited knowledge of video post-processing (my thing is photography, but I like learning).
I am adding a screenshot of what my render settings look like - I have no option to configure the "automatic" setting, nor am I seeing the bitrate (ditto in the Advanced Settings panel below).

I looked at FFMetrics but that's Windows only unfortunatley. Nor sure the metrics would make sense to me though :/

1) Cue Marc and his "h264 is bad for post" post

I don't mind people having to render H.264 files for delivery. That's a real thing, and it's not a file being used for post -- it's a file people are watching, which is perfectly legit. I just go to as high a bitrate as I possibly can.

I would use the Constant QP of 15, 15 and 15 (cue Jim and his screenshot)

Those are really good settings.

Though, I've found the file size significantly larger without a commensurate improvement in visible quality (at 400% pixel-peeping), so I landed on some less aggressive settings as a good balance between visible quality and file size.

nVidia users can download the Preset below.

https://www.dropbox.com/scl/fi/yubop2wm ... t7zd1&dl=0

(Change nothing but the Key Frames, setting that to 1/2 your timeline FPS.)