In 5' 2" and at my highest (last August) I was 330. For me that was size was "the biggest thing I could find in the store" I completely ignored anything with short sleeves- (because I have very thick arms) and anything form fitting. Pants were 26/28 and they were mostly snug.
After my 6 month medically supervised diet I am around 281 and pant size 22 fits me. All my old T-shirts are getting very big. I went shopping the other day to get some capris for post op- I found 2 pair of size 20. One I managed to squeeze my booty into ( and even button and zip) and the other I couldn't even get my leg down. I bought both because I know I will be able to wear them- one day!
So I think there are a lot of different factors that determine sizing like height, weight distribution, manufacture, and body style. We are all so different.
Ps. I'm totally jealous of overweight people with skinny arms- what I would do to wear a sleeveless shirt!!! My kids joke about me having bat wings now- I can't wait to see what they say 120lbs from now!!! I'm so self conscious of my dang arms!