IANAA but here is a wild guess; stars are limited to the sizes we see because they accrete from a swirling gas/dust. after they begin fusion (thousands of years

IANAA but here is a wild guess; stars are limited to the sizes we see because they accrete from a swirling gas/dust. after they begin fusion (thousands of years after) and photons are reaching the surface the nascent star begins emitting a stellar wind which pushes back the in-falling gas/dust which limits how big they can get. the important bit is how much more energy is in the stellar wind than the void it is pushing into. The early universe was hotter less void, solar wind had less advantage and more gas/dust continued to accrete. If ever a still growing star approached black hole mass, solar wind is extinguished and in-fall is in unimpeded forever. (for human values of forever) but I would go with what raattgift says over this wild speculation