It is strange what you are mentioning, years ago , when the Vietnamese eat lots of rice and lots of vegetables, much more than meat, everyone look like a walking stick , in fact, since it is a rare phenomenon, "beo tot" or " chubby nice" is a nice complement in Vietnam , the first thing one's friends greeting to the person is always "oh my God , why are you so skinny ? is something wrong ?"I used to be. Off and on for 10 years. My main problem was the high price of vegetables, the need to eat all the time, and huge weight gain and edema when I tried to go high starch. There is a ton of water-weight gain involved, and then the veggies do not have the caloric density so you get trapped into this never ending freeding frenzy. Just look at the frutarians who seem to think that eating 30 banannas per day is sustainable. The less starch you eat, the more you need to eat, the more starch you eat, the faster you will balloon up. (I gained 2 sizes the last time after about 3 months). Sumu wrestlers are vegetarians. You certainly do get enough protien to live on, but in addition you need to eat a ton of carbs.
Nowadays, the diet become more westernize , to a degree , lots of meat , and sugar drink over in Vietnam....and everyone look more chubby for real (or at least chubby only in comparison with the skinny of the past, not the ultimate chubby yet) and now, lot of cancers over there too .
So it is not the vegetable and rice or starch that promote the weight gain for sure, I can vouch on that with the proof of thousand years of starch & vegetable diet of the Viet.