Video quality different when recording alone than with several people

Video quality different when recording alone than with several people

For example, if I'm recording a video course and I'm alone in Zoom, the video quality is fine.
The video quality changes if I have several people in the zoom, then the recording is much worse.

I would like to offer recorded video zooms with several people for sale. However, the video quality is so poor that I can't offer this.

- Paid Zoom subscription
- I record locally
- Check mark for Video=HD => is active
- Apple device,  2-3 year old
- The video display during the meeting is also good.

If I only record myself, the quality is fine.
As soon as I have several people with me, the recorded quality is much worse.

No change to the settings between the two recording variants.

What could be the reason for this?
What can I do?

Thank you for your answers
Best regards
Manuela