Why are all the stars not initially the same size?
The initial size and density of the gas cloud is the determining factor. A dense cloud will have the gas and dust fall into the center much faster than a less dense cloud. As a result, once the fusion starts, the radiation is trying to push out really fast moving gas/dust and it can’t initially do so. So, the star continues to grow until it gets large enough that the radiation pressure is enough to blow away the in-falling material. Also, by that time, the initial cloud (at least in the vicinity of the star) is much less dense than it used to be.
