Who would win in a short wagon pulling race with a man sitting in the wagon. 40 10# chihuahuas all buffed up and trained for pulling or 8 50# strong dogs all buffed up and trained for pulling. My bet would go with the 50# dogs. The wagon would have to be on enough of a grade to prevent the dogs from reaching close to maximum running speed.
I’d go with the 50# dogs because of their longer legs. But I’d love to see 40 tiny dogs pulling a wagon. 
The smaller the team numbers, the easier it would be to manage and keep them working together and in line. The 50# dogs will win the day. You won’t get all 40 of the 10# dogs working together 100% of the time and that will cut their efficiency.
I used to see a disabled guy with a powered wheelchair out each day, catching the air and walking his tiny dog. Who was always, always straining at the limit of the leash. And thus looked for all the world like he was pulling the guy’s chair. Made me laugh every time.
I’d agree that there’s a scalability limit here. A vast number of tiny dogs would have too much efficiency loss to compete with a much smaller number of larger, longer-legged, heavier, stronger dogs. Just as that same 5-8 dog team could outpull even the biggest single dog of equivalent weight.