Screenshots from Fei Ren Zai, episode 49.
Fei Ren Zai used to be a Chinese webcomic but it got adapted into a small webanime. It is about the daily life of several supernatural Chinese entities (nine-tailed foxes, dragons, talking dogs) in human society and modern China.
I learned about this work from The Big Cartoon Wiki (and I strongly suggest you go check out their page for it). There is a specific weight gain episode centered around the male dragon character - the white dragon with a grey mane and blue eyes. As a result, I decided to check another episode to compare him slim with him fat.
Episode 49 is actually the dragon’s next appearance after the weight gain episode. I noticed as a result that his torso seemed a tiny bit thicker than usual, so I will have to check older episodes to make sure. Maybe he kept a bit of his flab underneath, or maybe the shirt is a bit more baggy than usual, but anyway, it is a really minor detail - you can consider this as his thin default form.