Chinchilla sizes

Been looking for a companion for my female chinchilla and seen a couple of females at my local pet store. Only trouble is they are about twice the size of mine. Mine is almost a year old. The pet store said that they are from the same litter and are around 15 months old but they are massive compared to mine. Not really sure what to do. I was gonna have another female chinchilla sectioned off in the same very large cage (3 stories) so that they could see and sniff each other to get used to each other that way without the immediate contact. I considered getting the two from the pet store as i thought it would be unkind to seperate them. despite me only wanting one, and the store having them for many months they weren't prepared to do a deal for the two so that they were cheaper.



I guess what im asking apart from your opinions is why mine is so small.



Thanks