In this case because he's doing a low-dosed cycle. But even if they temporarily shrink it doesn't mean he won't recover. hCG doesn't prevent damage to the cells in the testes and so even if they maintain size, that can't guarantee they'll respond well to physiological stimulation from LH/FSH post-cycle. I've had guys use low dose hCG throughout and seen no consistent difference in serum test levels 4 or 8 weeks post-cycle compared to those who don't.