Midsize and chubby/thick

In the first two pictures, I included what I consider to be midsize—someone who isn’t skinny but, when standing next to a very fat person, appears slimmer or not as big as they actually might seem (if that makes sense). I feel like this subreddit often features either very thin or slim thick or fat people, but rarely anyone in the middle.

Now, in the last two pictures, I wanted to ask a question: Where is the line between ‘thick’ and ‘fat’/chubby’? I’ve noticed that many people who are chubby but have a slim waist are labeled as ‘thick’ instead of ‘chubby.’ Is that characteristic what defines the line between chubby and thick? Or are there other factors that determine the difference?