Why do large dogs seem to like to swim more than smaller dogs?

i spent a lots of time by a river this weekend, and I noticed that almost all the dogs that were swimming in the river were large breeds. I saw several labs and goldens, a german shepherd, and even a young woman letting her bull mastiff swim. There were some smaller dogs, but none were in the water. It almost seems like in some ways, smaller dogs behave more like cats when it comes to water.