This information is based on Table 1 in Chapter 9 of the NEC, which specifies maximum raceway fill:
53% max for a single conductor in the raceway (i.e. - grounding conductor)
31% max for two conductors
40% for 3 or more conductors
The unused volume of air space within the raceway is needed for cooling of the conductors which tend to warm up due to conductor heating caused by current flow thru the circuit conductors. Overstuffing a raceway, aside from making for a difficult pull, would contribute to conductor overheating by reducing the cooling medium.