I guess it depends on how long he took to ask me out, and how much flirting he was doing.
If a guy was flirting with me but then never made a big move for a long time, I would start to think that maybe he's not interested in me, and maybe he's not flirting and this is just how he talks to everyone. Also if I can clearly tell he's flirting because he's forward about it, but then didn't ask me out, I might wonder if he's just trying to sleep with me, or if he flirts with a lot of girls and never asks them out. Another possibility is, if I knew he was flirting with only me, and he was really sweet and just seemed scared to ask me out, I might make the move and ask him out (I would never guess that he was waiting until he lost weight. I would just assume he was nervous).
So yeah, I think it's okay to wait but make sure that you keep flirting with her, and make it clear that you're treating her differently than you're treating other girls.
I would honestly admire that. You're working on yourself and there's nothing better than that. But try not to wait too long, some girls do not like to take the initiative of asking a guy out first so they become impatient waiting for you to ask and move on.
If your idea of losing weight is just to date multiple random girl, then that is not the main key to successful life physically and mentally. What should matter is, WHY are you doing it? Who are you doing it? Yourself! Realise that there are so many things you could do within yourself to make you feel content. When you're at the right moment (when you are at your goal weight), that is when you have no more doubts and just "go for it". In my opinion, you should be inspired with fitness through anywhere you can for now. Set the goals right. This might be your 2016 resolution.