If engines are hard to build, why not build a car 3x the size of a normal one, well you can but due to things like aerodynamics, etc etc you'll never match the speed or fuel economy of cars.

Same with chips, efficiency, speed, etc all depend on good design, and cutting edge factors, if the main reason your chip isn't faster is because of the distance between your L1 cache and your core is far, then having a bigger node process but bigger chip won't make it quicker.