I opine that the larger power, the more difficult it is to get the engine out through an HST to the wheels. I had a 14HP JD650, manual transmission. It had lots of umpf (for a small tractor), you could certainty stall it pushing or pulling too hard. I replaced it with a 25HP JD1025R. It does not have the [mini] brute force that the JD650 did. When you work it too hard, it makes really unpleasant noises which the engine turns and the wheels don't - so I stop doing that. But the 650 would either have moved the load, spun the wheels, or stalled. So I opine that the tractor manufacturers know this, and don't build bigger tractors which won't pull a heavy load well.