Wow I think more women would want to go out with you after they see this relation between you and your girlfriends' weights! It could be just pure coincidence. How many girlfriends are you talking about? And are you perfectly sure it's all of them who are losing weight? Sometimes, people tend to just count the hits and ignore the misses and make relations between things that aren't relevant to each other.

Or are you vegetarian or do you have a small appetite, or do you exercise a lot, and they follow your lifestyle and start losing weight too?

Anyway, the main thing is they aren't complaining, and they are happy. So why even fret over such an assumption? Unless you are absolutely curious or have a lot of time on your hands, I don't think you should worry yourself over this or see a therapist.