Going to a smaller SSD on the new mac

If you have less backed up than what you purchased for storage, yes.

If your existing backups are larger, then a smaller target storage device involves manual management of your contents, either once and short-term and only if you prune a whole lot and never expand, or involves long-term and on-going storage management particularly if you're trying to stuff 10 kilograms in a 5 kilogram sack.

And it's unlikely that macOS and apps are going to get smaller.

It's possible only if the amount of data that's being restored will fit on the smaller SSD. However, you should maintain a minimum of 50 GB of free space for optimal system and app performance.