As others have implied without stating outright, the rating of the light fitting is down to the amount of heat it can safely disperse.
Most of the energy a normal light bulb uses is wasted as heat and not light: say it's 90% as a rough guess. A 100W light bulb is therefore really a 90W heater and a 10W light source, and a 75W bulb produces 67.5W of heat and 7.5W of light. Your 20W energy saving bulb generates the same "light power" as the 100W bulb (10W) but only produces 10W of heat. It is therefore completely safe in a "75W fitting" which can handle a 67W heat source.
NB though you should be careful: the rating will be based on a standard light bulb's shape. If your energy-saving bulb is a different shape (as many are), and would come closer to the fitting, then it may provide localised heating which the fitting can't cope with.
As an extreme example, a very large energy-saving bulb might touch the fabric of a lampshade into which you put it, whereas a normal bulb would be 3 inches away. The surface of the bulb may get hot enough to ignite the fabric when in direct contact: so that wouldn't be safe, even though it was within the official rating.
But as long as it doesn't get much closer to the fitting, you're probably okay.