If you got them just recently, it is probably stress. Could also be smaller because you have a male:female pair.
Males are a lot smaller than females. My boy is about 1/2 the size of the other 2 girls, and is a fair bit skinnier. Milks are very hardy, so I wouldn't be too too worried.
Just make sure they're in a quiet area and that 3 sides of the tank are covered & they should settle in with some time.