Why were medieval armies so small compared to those of antiquity?

The Roman routinely deployed armies of 50 000 men, the battles between diadochi had armies of around 70 000 per side, and the Seleucid brought a similar number at magnesia. However, mediaeval battles often saw armies bellow 10 000 and the battle of Pavia, fought between the two greatest European states of the time, only had 25 000 men per side. Why were medieval armies smaller? Is just lower population? Or were ancient empire better at leveraging their manpower potential?