All other things equal, a heavily loaded engine is more efficient than a lightly loaded one. But tractors are not just engines. Weight has a huge effect on efficiency when we're talking about drawbar power. A small, light tractor is going to have more slippage than a heavy tractor with big tires. To compound matters, the smaller tractor may have to pull an implement at a lower speed than the big tractor. The slower the speed and the lower the gear ratio, the more power is lost to slippage.
In the example you give, pulling a four bottom plow with a typical 60 hp tractor is going to result in a lot of slippage. The 90 horse tractor is going to pull that plow easily and in a faster, more efficient gear. Of course, either tractor can run the hay rake, and the smaller tractor will do it using a bit less fuel. But on average, I'd say the bigger tractor is going to be more efficient.
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