Over the years, I've realised that a) telling people things they already know deep down and b) telling them things that they can realise and learn all by themselves is pointless.
Your daughter learned something, not through the interaction, but when she tried on clothes and was upset and they didn't fit. The best thing to do is be very neutral and let her come to her own motivation and conclusion about what to do next.
No-one suddenly gets thinner for someone saying 'you are a bit pudgy, don't you think you should eat less', They get thinner when their own internal motivation for change builds, and the environment supports that change- so by not buying unhealthy food, having positive enjoyable experiences with healtheir food or activities, and so on.
I also think young people do best when their Mum is firmly in their corner, whatever happens- so if she needs bigger clothes right now, I'd help her buy new stuff IF she asks you to help her. If she does not, don't say 'do you think you need bigger clothes' Feeling like your Mum basically disapproves of your body is very difficult- none of us can help having uncharitable thoughts about their behaviour, but you can offer to help in positive ways without criticising her or pointing out her weight gain.
Finally, a lot of young people have decided not to wait til their bodies are perfect, but to go out wearing what they like and what's in fashion. I would encourage that side of her life very strongly- any social connections, getting out and about, joining back in with life, and it does not matter if she goes out looking how you think she looks awful as it's better to be out and about for her MH.
I wouldn't lie, I'd say, I have to be honest, that's not the outfit I think looks the best, I like X outfit, but I'm not a young person, and laugh. Then stock up on great food, go for some walks with her to chat, and let her find her own way with her weight. You can't police or shame people into being thin from outside, it has to come from within.