In NH3, there are 4 electron pairs (3 bonding pairs & 1 lone pair) in the outermost shell of N. Phosphorous has a bigger size and larger p orbitals (2p vs 3p). This reduces the overlap of hydrogen and phosphorous orbitals compared to that of NH3. Bond angle is depend on various factors. In that electronegativity plays a major role. N is more electronegative than P due to this, N will attract hydrogen more than P thus bond length of N-H is less than P-H. In NH3 Since the central atom is more Electronegative, it attracts the shared paired of electrons. They experience a strong repulsion due to electron – electron repulsion force.