If he hadn’t shrunk Dean back down, they’d have a lot of work to figure out how to get him back to normal.
Also, RIP Rufus’s cabin. It wouldn’t survive the attempt.
Sam and Dean couldn’t interact with each other reliably at that scale difference. They’d need Bobby to help soften the blow between them.
The curse Celeste used on Dean was not the same as the one that she used on Sam. This one was a standard witch spell.
Though in Brothers Apart we see her angelic soulbound curse used most often, there are other styles and types of magic and curses that can change sizes, either larger or smaller. The reason she used hers on the borrower population was because the angelic grace imbued in them strengthens the host body, and she’s growing an army of hosts and in particular, looking for a strong one for her brother to use.
Dean’s curse was easily lifted and removed. If they had to wait until later, a simple cleansing spell would have worked.
It would be painful, though. Crunching down to normal size all at once.
You will see other size changes happen in the BA multiverse.
Brothers Apart 