The answer to this question depends on what you mean by "breed". If we are talking about purebred horses, then there is no single breed that can claim to be larger than any other. However, if we include all breeds and cross-breeds together, then it's a pretty safe bet that Clydesdales would take first place in terms of size. They're not only the tallest breed of horse but also have the longest legs for their height. In fact, they've been bred specifically to produce large offspring!
The next biggest breed of horse is probably the Thoroughbred or Standardbred. These two breeds were originally developed as racing horses, so naturally they tend to be very tall and long legged. But even though these breeds may look like giants compared with some of the smaller breeds, they still aren't quite as big as Clydesdale stallions.
According to Wikipedia, the average weight of an adult male Clydesdale is 1,200 pounds. That makes them roughly twice as heavy as the heaviest known thoroughbreds.
In summary, here are the top five most massive breeds of horse:
1) Clydesdale Stallion - Average Weight 1200 lbs
2) Thoroughbred Horse - Average Weight 1000 lbs
3) Arabian Horse - Average Weight 900 lbs
4) American Quarter Horse - Average Weight 800 lbs
5) Andalusian Horse - Average Weight 700 lbs