The compressibilities of ScS and a series of Sc1-xS metal defect structures (x > 0) were studied using a diamond anvil cell and synchrotron radiation at pressures up to 73 GPa. The results show that the bulk modulus increases with x, in contrast to the usual view that a less dense material is more easily compressed. The results are in qualitative agreement with existing band structure calculations designed to probe the mechanism of the ready metal atom loss from ScS at high temperatures. They are also in agreement with new calculations, both of the tight-binding and the first-principles FLAPW type. Numerically the latter show an increase in the bulk modulus of 30% between ScS and Sc0.875S and are in exact numerical agreement with the experimental results.