why is my faverolle so small compared to the others

[Bellapollo]

hey guys, I have 2 faverolles the exact same age, from birth they looked the same and until about 2 months however the past month all my other chicks have gotten big and put on weight but this one has not, its the same Size as 2 months and we all know with chicks how much a month makes a difference in growth. the photos dont do it justice but you can tell its smaller then its twin and the twin is the same size as †he other chicks. in the second pic its standing up tall but its obviously thinner and smaller. why is this??

 

[Wyorp Rock]

Does she always stand like that?
No one can really know "why", but if I had to guess, she may have some type of developmental issue. Some chicks even from the same hatch can be quite different and grow at different rates. Could be genetics or 100 other things.
You can try offering her poultry vitamins once a week to give her a boost, see that she is eating the chick starter well, monitor her for lice/mites.
Time will tell how she does, she may just be slower to mature.