Hi
Have found that to be a very common event in various types of fish . Most noticeable in batches of fry. From what I gather they release a growth hormone into the water stunting the weaker members of the group.. Only way I've found to combat it was frequent water changes . With some types of fish such as Bettas it requires like 50 percent per day!!!. I have found in GF that the smaller will catch up over time though they also seem to remain smaller . gary