Stop multipass
sudo launchctl unload /Library/LaunchDaemons/com.canonical.multipassd.plist
Edit the config
sudo vim /var/root/Library/Application\ Support/multipassd/multipassd-vm-instances.json
you can edit “mem_size” there (it’s in bytes)
Start it again.
sudo launchctl load /Library/LaunchDaemons/com.canonical.multipassd.plist