There's a lot of bad information on that. It takes a BIG horse to run a fast 6 furlongs.
People tend to believe exactly the opposite.
A horse uses more oxygen per yard traveled over a sprint than in a route race where he gets a chance for breathers. The good sprinter needs a big chest and big lung capacity. Good trainers in scheduling a blowout for a horse pick a time when the humidity is right. A horse sucks in air and when he's getting something close to pure oxygen, his blood is red. As he moves on out and the oxygen burns up in his system, his blood gets pinkish and then blue.