Pretty interesting take from TheFakeWeeb on YT:
Sukuna's wounds could be due to him also condensing his domain to adapt it to Gojo's.
As Kusakabe says, doing so should be impossible since a domain with a mass inferior to the one using it should result to the user own collapse. Though, as Choso pointed out, Gojo must have learned something in the prison realm Sukuna isn't aware of.
Basically, according to this hypothesis, Gojo tricked Sukuna by forcing him to do something that would inevitably crush him like what happened to the Oceangate when touring the Titanic, causing his wounds and his DE collapse.