I think its just distortion from the angle the pics were taken from. 2 of the loaches are larger ~5", the other two I've only had for a month or so and are much smaller - 1.5". The little guys are good shape, just look tiny next to the two older ones.
Deb they are beautiful and so healthy looking! Your tank looks great too from what I can see. I'm starting up my tank after the summer and I'm looking for loaches to get along with cory cats as I love them both. I love this thread!