If you're buying an SUV for, among other things, its towing capacity, that extra power is vital. From that, due to the laws of physics, there inevitably comes lower gas mileage. My '04 Navigator gets about 15 mpg on the road when it's not towing my 7,500 lb. boat and trailer rig. When I'm towing the rig, mileage drops to about 7.5 mpg. There's just no way a hybrid will work for me.
The $9,500 premium a purchasers pays for a Prius is just too much to be economically recoverable at a $580/yr. gas savings. Not accounting for the time value of money (more about that later), it would take 16.4 years to recover that $9,500 through fuel savings. However, if that money was invested at an 8% annual return, it would double in about 9 years, meaning the gas savings will never enable the vehicle's purchaser to recover the $9,500. Even worse, the batteries need to be replaced about every four or five years at a cost of about $4,000, which is just about the price of a rebuilt engine. One of these things is nothing but a head up the behind feel good device for proving to one's neighbors how environmentally good one is.
There is no 'law of physics' that dictates that an engine which is capable of supplying more power would have to be significantly less efficient than one which isn't. As a practical matter, that is almost always the case because engines have, for over a century, used a very inefficient method of controlling power output (the throttle).
When a naturally-aspirated (i.e. non-turbocharged/non-supercharged) engine is running, it will try to draw in air faster than it can flow through the air intake. This produces a partial vacuum. To limit the power output, the air intake is constricted by the throttle. Thus, when the engine is throttled down, it draws a deeper vacuum.
In a normal car, almost 100% of the energy used to draw a vacuum is wasted (some may be used to operate things like power brakes, but that amount is really tiny). The lower the power demand on an engine, relative to its capacity, the more energy it must waste in the throttle.
To get an appreciation of how much energy is wasted in a throttle, recognize that when you downshift so as to use the engine to slow down the car, it's the throttle that's absorbing all the car's kinetic energy. That 'drag' you feel when you downshift is drag that the engine has to overcome while you're driving, and represents 100% wasted energy.
Replacing the conventional throttle with something better would go a long way to improving the efficiency of large engines.