How massive were Population III stars compared to Population I stars like the Sun?

They ranged from 100 to several hundred solar masses ($M_{\odot}$).

Astrophysical consensus indicates that the initial generation of stars were true behemoths, orders of magnitude larger than stars observed today. Because the metal-free gas clouds lacked the efficient cooling mechanisms that promote fragmentation into smaller pieces, the gravitational collapse favored the formation of extremely large stellar cores. Theoretical predictions place the mass range of these Population III stars typically between 100 and several hundred times the mass of the Sun. Some extreme models even suggest that the absolute first object could have been a single star exceeding 1,000 solar masses, reflecting the unique, high-mass formation physics of that era.