Significance Macromolecular crowding influences protein−protein interactions via hard-core repulsions and chemical interactions. Scaled particle theory predicts that the effect of hard-core repulsions depends on the shape of the protein complex, and simple ideas about chemical interactions predict a dominant role for the chemical qualities of the protein surface. The theory predicts that a collapsed, ellipsoidal, dimer will be stabilized by hard-core repulsions, whereas a less collapsed, side-by-side, dimer will not. We applied scaled particle theory to two dimers with nearly identical surfaces but different shapes. Our results support the predictions; crowders that interact primarily through hard-core repulsions stabilize the ellipsoidal domain-swapped dimer more than the side-by-side dimer, whereas crowders that interact via chemical interactions have the same effect on both dimers.