This is something you will see no matter what species of fish you raise. There will be some big, some small. Those stronger will outcompete the rest for food, and add the effect of phermones secreted to impede growth, hence the size differences. Then there will always be those that are especially small and do not grow even if you separately raise them...the runts.

The best way around this is to do regular culling to remove the deformed and runts at early stages, then also separate according to size.

There is nothing wrong with what you encounter.