Migration copies your problems too, and some of them are 40 GB

Migration Assistant is very good at what it does, which is moving everything. That includes eight years of caches, the Docker disk you never pruned, and support folders for software you stopped using in 2021. A new Mac is the one moment where clearing them is free.

Clear these on the old Mac first

1. Docker's virtual disk, after pruning. It is frequently the largest single file on a developer Mac.
2. Build output and package caches, all rebuildable on the new machine.
3. Old device backups, one per phone you have ever owned.
4. Downloads, which is installers and disk images almost all the way down.
5. Virtual machines you have not booted in a year.

du -xh -d 1 ~ 2>/dev/null | sort -h | tail -20
du -sh ~/Library/Containers/com.docker.docker 2>/dev/null

Ten minutes here saves both the transfer time and the space on the new Mac, and the caches rebuild themselves the first time you use each tool.

Migrate, or start clean?

For most people full migration is right, and the preparation above is what makes it right. A clean start is worth it when the old Mac has been through several macOS upgrades and carries software from a job you no longer have.

What migration will not fix

A second user account created by a previous migration comes across as a second user account, which is one of the largest things that can hide on a Mac. Check the Users folder on the old machine before you start, not after.

Space on the new Mac

The new Mac needs room for everything coming across plus its own working space, so a transfer to a machine with the same size drive is tight if the old one was nearly full. Clearing the old Mac first is what makes the arithmetic work, and how much free space a Mac needs sets the target to leave.