Bristlenose Catfish, Id?

[JSmails]

I bought 2 bristlenose catfish 6 months ago. Both were very small. One has trebled in size but the other has not grown at all. I am confused as to whether this is because she is female or actually a different species as she is not as round as the larger one and also has a white patch on the tip of her tail which I cannot find in any of the photos of bristlenose catfish.