My dog's rib cage is extremely flexible. Is this normal?

I have a dog, Zeke, who is approximately a year old. We adopted him from the Humane Society, so we don't know what breed he is specifically, but we were told he is a black lab mixed with something. He is a large, hyperactive dog, but his rib cage is very narrow, and extremely flexible. By applied the minimal amount of pressure, you can compress his rib cage significantly. He has never had any issues breathing or eating or anything, but I am simply wondering if this is normal or perhaps an indicator of what breed his is mixed with. Thanks!