Measurements matter to me more than the scale, but the scale is still a tool for progress...it's just not THE ONLY tool one should be using.
There's a bunch of reasons why you could go down in size but not in weight: you could be holding onto water, you may have changed your body with exercise, etc.
Muscle takes up less room than fat, so it's possible to weigh the same as someone else and be smaller. I like to talk about the time I purposely gained around 5 pounds and yet found that my clothes fit better. I gained while lifting so I build some muscle there