One of my chicks is smaller... Is it normal?

So, a story must be necessary.
I got three chickens, three months ago (they were a day old at the time). Not sure what the breeds are called in us, but in Denmark and in Danish, I got a New Hampshire (a chubby neurotic chick)and two Helhvad (slim and calm chicks).
A week later, I lost one of the Helhvad chicks (my heart broke! ), to some kind of a bird.
I was told by the seller, that I had to have at least three chickens, so I went back to the same store and bought one more.
To my big surprise, the chickens at the store seemed small compared to the ones I had at at home, even though they were from the same brood :/. I ended getting a Skalborg this time.
When I got home, there was no doubt, that my chicks that I got a week before, we're bigger than the new one. Also, at the store I noticed that they didn't have clean water or feed, and the box they were in, was covered in poop.(could this have anything to do with it?)
My concern is now, three months later, the third new Skalborg chick, is still smaller than the other ones. I thought that in time she would catch up with the others sizewise.
It doesn't seem like she's sick; she the most loving of them, loves to cuddle and sit at my lap, loves to run around compared to the other ones, doesn't panic when I pick her up to cuddle with her inside the house (the New Hampshire HATES to be away from the two amigas).
What are your opinion? Will she never catch up on the size? Is it possible that the one week was enough to make a difference?
(Btw, she doesn't eat everything that I serve, but then again, all of my chicks are very picky compared to what I've read. My fiancé says it's because I've spoiled them with "fancy" food )

 

Ok, great to know e they all get along, my Hampshire panics if she's not with the others, and they love to cuddle with each other, and always making the comfy sounds when they are around each other