I'm sure the reduction in cut by 1 foot plays a big part, but remember, not all cutters, even of the same size, use your engines power the same way. The gearbox, blade design, deck design, and even the way the blades mount all make a difference.
As an example. Take 2 blade designs. Let's say they're identical with the exception of where the offset from the blade mount down to the cutting edge is located. Blade "A" offsets the blade downward immediately after clearing the blade holder. Blade "B" stays level all the way out until just before the cutting edge. If you go to your local supplier (TSC, Rural King, etc.) you can find both types. Blade "A" will have a long unsharpened section level with the cutting edge. Blade "B" will have the same length, but all the unsharpened part will be at the blade holder level. Everything at the cutting edge level is sharpened. Which takes more HP to run? Blade "A" comes into contact with the grass, brush and other material with not only the cutting edge, but also the unsharpened section. Blade "B" only has the cutting edge in contact with material, (unless of course you climb on top of that big rock or stump /w3tcompact/icons/smile.gif).
Most 4' cutters have blades so short that they fall into the Blade "B" category. When you get into larger 5' and 6' cutters you really notice the difference. This factor, in addition to the blade flex factor is why many upper crust manufacturers use shorter blades and larger blade holders on their bigger cutters.
This is a long way of saying that not all cutters are equal. Just because your neighbors Brand X cutter bogs your machine down, doesn't mean Brand Y of the same size, will.