I dunno. It sounds like she knows that hot chips and fizzy drinks can make you fat. Which they can. But not one serve each on one particular day so I would explain that to her and say that eating them occasionally will not make her fat but if she ate a lot of them and a lot of other unhealthy food and wasn't getting enough exericse then yes, that could make her put on weight. I'd also explain that you can eat a lot of healthy food and not get fat.

I'm saying "I dunno" because I think it's too early to tell whether she's heading down the eating disorder line or not. The jumping on and off the scales thing could just be to see whether what she'd eaten that day would make her weight go up. That could be a child's natural curiosity (just like I will sometimes weigh myself before and after a poo OR it could be a sign that she's getting overly obsessed. Hard to say.