Jennifer Lawrence was slated for being too ‘curvy’ for the film, and the District 12 training suit definitely emphasises her curves but, lets think about this for a second…
Yes, in the book it mentions that she is always hungry and prioritises Prim’s nutrition over her own but, at the end of the day, she’s a hunter, so she has access to food, including meat.
The character is 16 also, right? I knew some people (back when I was 16) who hadn’t eaten much for a while and were really skinny. they hit 15/16/17 and suddenly their bodies changed, but they didn’t increase the amount of food they ate, they just ‘blossomed’ as it were.
Also, both Katniss and Peeta are offered a wide-variety of foods on the way to the Capitol and while there, so they would put weight on then also.
And yes, this may get some critical feedback, but it is just my opinion remember.