Read the label carefully.
The majority of "reduced fat" foods of all types have some type of sugar added so you are, in some respects, trading fat calories for carb calories (but the low fat version may, in fact, be somewhat lower in calories). This certainly goes for peanut butter, which usually adds corn syrup to make up for the fat.
Peanut oil is a fairly healthy fat but they hydrogenate it to make PB solid at room temp. Thus, many brands contain tras fats. The more fat in this kind of PB, the more trans fats you consume.
IMO using less full-fat PB is probably a healthier choice. Non-hydrogenated PB even better.
Less is not more. More is more and more is fabulous.