is this size difference normal?

Ignore the sunbathing pullet. That's the mother hen and her three chicks she hatched 18-19 days ago. Two of the chicks are much larger and have much better developed feathers than the third one.

Since we don't have roosters and are only currently walling up, neighbours' roosters must have been the one to fertilize the eggs. My mother says it's probably due to them having different fathers that they look so different but I just wanted to ask more knowledgeable people.

We have a flock of 5 chickens currently:

1. This mother hen and her chicks

2. A laying chicken

3. An older pullet

4. The younger pullet seen in the video

5. The younger pullet's cockerel brother (who you can also see limping for a second in the video, I don't know why he's limping but there's no scab on his foot and we originally quarantined him for three days but he seems to be better so we let him out today - he doesn't seem sick, just limping on one leg)