We use a recently developed constant-pressure molecular dynamics method for nonperiodic systems to study pressure-induced structural transformations in medium-sized silicon nanocrystals, where the kinetics is experimentally known to be bulk rather than surface dominated, choosing Si-705 as a representative example. Pressure is applied and tuned through a liquid described by a classical potential, while the nanocrystal is treated within a tight-binding scheme. Upon pressurization the nanocrystal undergoes a structural transformation which starts at the surface and gradually propagates into the bulk core. The high-pressure structure is disordered and metallic, with an x-ray diffraction pattern compatible with both the ideal beta-tin and simple hexagonal structures. Strong similarities with a recently calculated high-pressure phase of bulk amorphous silicon are evident. Upon pressure release, the original diamond structure is not recovered and a high degree of disorder persists.