Fitting a bigger intercooler won't raise power on it's own unless you up the boost a little to compensate for the higher amount of cool, dense air going into the engine. The boost should drop a tiny bit if the intercooler is doing it's job properly, yet the engine will make the same power. It just has to push less air to get the same result. Hence a power increase is seen when the boost is brought back up to the same level as before, because you're actually getting more air (denser) flowing into the cylinders.
The above is only true for comparing two intercoolers in exactly the same condition, so isn't the case with most of our cars.