I don't think it's just a weight thing, because I STILL have this happening to me. I've even had the lovely experience of knowing someone for YEARS only to have them forget my name, and I chalk it up to my still not having a clear identity. Even though I've lost all of this weight I'm still trying to find myself and be more of a "people person", you know?
So I usually don't take it personally. It's not always a bad thing to be able to blend in and, let's face it, some people are just distracted and oblivious
Also, someone recently told me something interesting. They said that people tend to like and remember those who remember and use THEIR name and things about them, and it's so true. I know that if I meet someone new who makes it a point to use my name the next time we meet, or mentions something personal about me, then I remember them as well and my estimation of them goes way up.