The source for the sizes of Secretariat's and Sham's large hearts was University of Kentucky pathologist Thomas Swerczek, and appear to be limited to his experience. Swerczek performed the autopsies on both Secretariat and Sham. He's quoted extensively in Marianna Haun's book, "The X Factor".
Here are Swerczek's statements about Secretariat's heart size (and Sham's):
"He was completely out of everybody's league."
"We just stood there. We couldn't believe it. The heart was perfect. There were no problems with it. It was just this huge engine."
"I looked at it microscopically. It was perfect. It was just tremendously large."
"'It's humongous.' The other vets didn't have the benefit of looking at hearts every day, so they wanted my opinion. They obviously thought it was very large, also, but they wanted to make sure they were seeing what they thought they were."
"It is, by far, the largest heart I've ever seen. There is nothing to compare it with. The closest one was Sham's heart. I don't know how many years later it was until I did that autopsy, but when I looked at it I immediately thought, 'My God, this is the second-largest heart I've ever seen.' But it was pathologically enlarged. It weighed about 18 pounds."
"I didn't weigh Secretariat's heart, but I could visually see that Sham's heart was smaller, so I estimated Secretariat's heart to weigh approximately 22 pounds."
It's stated in the book that the "normal" heart size of a thoroughbred is 8.5 pounds, and here's a table of large heart sizes from the book:
Eclipse 14 lbs
Phar Lap 14 lbs
Secretariat 22 lbs
Sham 18 lbs *
Star Kingdom 14 lbs
Mill Reef 16.94 lbs *
Easy Goer 15 lbs
Althea 15 lbs
Key To The Mint 15.8 lbs
Killaloe 12.9 lbs
Tulloch 13.5 lbs
* Some pathological enlargement (can add 2-3 lbs, or more, to heart weight)
+++++++++++++++++++++++++++++++++
I think that heart size, like stride, isn't really a useful way to compare horses. They're just factors that may help explain a horse's ability. It's better to look at what the horse actually did on the racetrack.
