Systematic crystal structure refinements from powder X-ray diffraction data as well as density functional theory calculations demonstrate that the silicon clathrate II Si(cF136) exhibits a lattice contraction as Na is introduced solely into the Si-28 cages. When the Sin cages, in addition, begin to be filled with Na, a contrasting lattice expansion results. The nonmonotonic structural response to filling is an indication of markedly dissimilar guest framework interactions for Na@Si-20 and Na@Si-28.