Pully size, imagine 2 systems A,B both transmitting power, both 1:1 but if OD of pulley is 50mm on A and 100mm on B, How do we choose optimal size?

All searches resulted in ratio calculations, which makes sense, my question relates to creating a servo driven spindle with 1:1 ratio with timing belt. Is increased belt surface engagement important since its a toothed belt? Also system A and B are examples only.