How did Taylor Swift gain weight? She looks so much healthier and happier now?

I am naturally very skinny like her (on the right) but i want to start eating plant-based/natural foods and exercise every other day. I’m scared if i do, i will end up looking very very skinny. How did she manage to not gain too much weight and stay healthy at the same time. How much weight is healthy weight? 

18 gibbs 202020-07-30T19:35:12Z

Frankly she looks 100 times better in the 2015 photo