All four of the molecules have tetrahedral geometry. Therefore any net dipole from the C-H, Si-H, etc., bonds will be canceled by symmetry and the molecule will have no net dipole. Since there are no dipole-dipole interactions, the only intermolecular force holding these molecules together are London dispersion forces. The strenght of these dispersion forces depends on the polarizability (not polarity) of the molecules. In general, larger molecules are more polarizable and therefore they have stronger dispersion forces holding them together. For example, this explains the trend in melting points from I2 (solid), Br2 (liquid), Cl2 (gas), and F2 (gas).