Why a huge difference in size/development of hatchery chicks?

Even a tiny, temporary setback in health can show massive differences in chicks. A small wound, a fleeting gastrointestinal issue, a slightly chilled night, a missed feed, can put a bird behind its peers developmentally. It may never catch up in some cases. Once a chick gets behind it tends to stay behind.

This case could be due to a lot of things. A setback as mentioned, varied genetics, different feeds from hatching onward (until you got them), lack of sunlight, exposure to a different disease/ vaccine than the other batch, all of the aforementioned, etc. It's really impossible to say for sure. It could even be a gender difference though that's not the likeliest at this point.

Personally I avoid hatchery birds like the plague I have found them to be. You will get some good-enough ones, in all likelihood, and if it's a great hatchery you may be entirely happy with them, but I have found far too high an incidence of every problem with them. Behavioral and health issues abound. In fact in my experience they're guaranteed. This is not an issue for you if you only want them for a limited purpose, but I don't like fostering birds which are mostly not worth breeding, which often harbor vicious or otherwise negative tendencies, bad genes, always sub-par health, etc.

My experiences though may be worlds apart from yours. Best wishes with them.