I strongly believe that Russia is preparing a more serious response. They just were not prepared for this, so didn't have defenses in place. Better to retreat and cede some territory than wasting troops in a losing battle. Since their land is so large, they can still come back later and reconquer.
Also, Ukraine can't use that many troops to actually fortify the region, given that they are also under heavy pressure at the more southern fronts.
And, 130 square miles is tiny compared to all the territory that Russia got so far.
Well, I believe the biggest win about this is for morale. Russians get scared because the war is now in their country, and Ukrainians get a buffer zone.
Ukraine decided to walk into Russia at a place where the border is not protected very well