While higher displacement usually means more horsepower, this isn't entirely true. Many, many other factors go into this, such as number of cylinders, type of engine (2 or 4 stroke, gas, diesel, propane, or other), super/turbocharged, etc.
For example, the 4.0 inline-six gas motor put in many Jeeps made 180 hp, but the 6.2 V8 diesel in my 1986 Blazer only makes 143 hp.