In the USA, the main drag racing sanctioning body, NHRA, limits engine size to 500 cubic inch V8, with pushrods and 2 valves per cylinder for Pro Stock, Funny Car and Top Fuel classes. In the case of Pro Stock, these are "crate" motors built by Chrysler, Ford, and GM. For Funny and Top Fuel, the engines are custom built. All are similar to the Chrysler "hemi" 426 cubic inch engine, but not based on it. Again, note that this is by the rules. Pro Stock class is normally aspirated and 1/4 miles times are around 6.8. There are supercharged and turbocharged street looking type drag racers that are faster, but these run in different classes.
The less popular IHRA allows for the larger engines, up to 800 cubic engines, called "mountain motors".
http://en.wikipedia.org/wiki/Pro_Stock
In the Lemans series, normally aspirated gasoline engines up to 8.0 liters are allowed, smaller if there is forced induction (turbo charger for example).
Although the power per liter is lower with these bigger engines, the power per weight isn't. As an example, the 7.0 liter, normally aspirated, pushrod V8, 505 hp, engine in a Corvette Z06 is lighter than the 3.6 liter, turbo charged, flat six, 480hp / 520 hp in a Turbo Porsche 911 / GT2. In the case of the Z06, 505hp is the guaranteed minimum, most make around 520hp. I don't know if the GT2's power is similarly understated. The Z06 weighs about 3150 lbs, 200 less than the 3350 lbs of a Subaru WRX STI (however you could buy 2 STI's for the cost of the Z06, and 2 Z06's for the cost of the GT2 with money left over).