Tow capacity ratings

I'm pretty familiar with tow capacities in calculating them. What I'd like to know is that makes the difference in rating between a Class III vs Class IV on the tundra? I have the 4.6L crewmax so is it just the engine size or is there something extra as far as the receiver is mounted to the frame. I was looking at ratings of the Tacoma and the tow capacity is approx the same utilizing an even smaller motor. How can that be. I just wanted to have further understanding in this area.

If there are no differences receiver wise between a 3 & 4, could you technically tow more as long as you're within GCVWR? Please excuse my ignorance.