That's exactly my problem, except that my system is old and upgraded. But it's exactly that, a smaller SSD than the HDD, even worse cause I installed a clean Win10 on the new SSD, while keeping the old OS on the HDD.
I always get the UI for the OS choice, but I haven't used the other since the clean install. Only storage room on disk.
Now, after hours of pursuing this "miraculous" disk shrink (if MS expects leigh people to get over such troubles as the ones that frustrated me), I give up. I finally shrank the partition, now reagentc won't work. As if there's no WinRE on my Win10...
Screw KB5034441, screw my unpartitioned SSD, screw this mentality tendencies to delegate responsability and assure income. Screw all this!