Transferring 275,000 emails from M1 to M4 MacBook via mbox file

I am transferring data from an old (M1) to a new (M4) MacBook. This includes a local mailbox which contains 275,000 messages. I tried to export this to a local mbox file and transfer it, but only 50,000 messages appeared on the new machine. In some old forum messages I've read about 2.15GB size limits, but I'm not sure if that's the problem given these are both relatively new machines. Is there any way I can do the transfer without having to split up the mailbox into smaller files. Thanks!

[Re-Titled by Moderator]

Original Title: mbox export size limit?