> I don't know enough about horses but a heavy horse (Shirehorse, Suffolk Punch or Clydesdale in the UK) can drag quite a decent load.
A heavy horse also needs to eat a lot more than a normal horse.
A heavy horse also needs to eat a lot more than a normal horse.
My father, born in 1930 in Minnesota, told of watching farmers bringing out a team of horses to help pull stuck tractors out of the springtime mud -- it wasn't only about their not-insignificant strength, but their superior traction. In his teens -- which would have been in the forties, he worked with teams of horses, which seems oddly late in the 20th century, but he did, as did my paternal grandfather. The terms for the various parts of the, to me, very-complicated-looking harnesses were as fresh in his mind in his eighties as they had been in his teens.
Now, can our nag drag enough fodder to do the job? I suggest yes.
But the question here is how far the horse can go on a single load of food. We're purely trying to convert food into work via the horse.
And if you assume that the conversion of potential energy stored in the food to actual energy produced by the horse is less than perfectly efficient, then it will always be better to avoid doing work than to try to make up for doing extra work by starting with more food. That would be true even if starting with more food didn't require doing even more extra work to move the extra food.
Improvements in strength aren't helpful to this problem; what you need is higher efficiency.