If you still have your old iPhone, I would recommend that you log back into it and delete as many large videos as you can.
It has been my experience that:
- WhatsApp backups don't complete if they are very large. This was several versions ago.
- WhatsApp restore to new iPhone does not copy everything, even when the latest WhatsApp version is installed on both phones and I did a complete backup to iCloud on the old phone immediately before restoring to the new phone.
The best success that I have had transferring my WhatsApp messages to a new iPhone is when I cloned the entire phone.
Note: When you do this, open WhatsApp on the new iPhone; not on the old one.
Howard