A lot of the times the perceived differences between mainland Chinese and oversea Chinese come from differences in fashion, styles, make-up, and facial expressions rather than actual phenotype differences.

Mainland Chinese students especially guys tend to have this very old-fashioned bowl-cut which tends to exaggerate their facial width, and combined with a high percentage of myopic students wearing thick glasses this tends to create a rather stark visual difference between them and oversea Chinese which tend to have better hairstyles and more often than not wear contact lenses rather than glasses. However if the two are having the same style than the differences would be much smaller almost negligible.

And there's also the issue of weight. Mainland Chinese culture tends to put more pressure on women to stay in shape and much less aesthetic pressure on men, hence the reason why there're more overweight men than women in China; and for overseas Chinese I feel it's exactly the opposite, the aesthetic pressure on men is higher than that on women. Hence the reason why you see more fit small-faced ABC guys compared to mainland Chinese guys, but for women it's the opposite.

And Beijing is a typical example of how weight influences phenotype. According to statistics Beijing ranks the top among Chinese cities in terms of obesity, with nearly 25% of Beijingers being either overweight or obese. If you've ever been to Beijing you'd surely notice that the breakfast in Beijing is very high in carbs and in sugar, for instance fried doughs the size of my small arm, or sweet deep-fried pancakes twice the width of my face, not to mention the various types of steamed buns, noodles, cakes, animal organs cooked in deep oil, etc. No other place in China has as heavy a breakfast as Beijing. I've watched many online streaming from different parts of China that's why I'm quite accustomed to the phenotype differences among Chinese, but the food differences are equally striking. I remember I've watched two ethnic Ayni / Hani girls from Yunnan having breakfast and their version of fried dough was like only 1/3 to maybe 1/4 the size of the one in Beijing, and when they ate it they dipped it in a bowl of chili sauce as opposed to dipping it in soy milk. And they also drank a type of vermicelli soup with vegetables. And I've watched two ethnic Hmong / Miao girls from Guangxi having breakfast and their breakfast was a small bowl of tea porridge or tea congee. That's how the South Chinese stay in shape.

Really when talking about Chinese food being heathier than western food one must exclude Beijing food from the list cause that's definitely not the case.