A full circle crank occupies more space within the engine and therefore increases the primary compression (this is the compression when the piston is on it's down-stroke and the bottom of the piston is compressing the fuel mixture against the reed valves before the piston travels down far enough for it to enter the combustion area). This is simply because there is less empty space in the engine bottom end.
In reality you will not see much noticeable performance increase unless the engine is highly tuned. However because these cranks are designed to be used on highly tuned engines they tend to be much stronger than other cranks.