How a long back affects a horse's stride?

My horse has a long back. He is also very muscular and stocky. His canter is like a rocking horse. I wouldn't call it choppy, but definitely like I'm on a rocking horse. Takes some time getting used to, that's for sure! My husband's horse has a long back too, but his canter is smoother than my guy's. But he's also a thoroughbred, where my horse is a stock paint. I'm wondering how much of it is the way they're trained...hmmm....hope you get some good answers, cause I'm curious too!