you're not the only one
i'm naturally very slender and a lot of female characters are very curvy. i feel like i wouldn't fill out cosplays as nicely as i'd like to.
try gradually making your portions bigger, loading up on proteins and snacking often
a calorie counter can help too. especially if its one that breaks down the amount of carbs, fat and protein you've had. you can usually input your weight/bmi and your goal and it'll tell you exactly how much you should be eating, mine has an exercise tracker too
obviously you're gonna wanna keep up with the work outs if you want to tone too
chocolate milk and peanut butter will be your friend haha
good luck~ (: