I think the difference is in our microbiome. A study I heard about showed that the gut microbiome of recent immigrants assimilates to that of their new country very quickly, I think it was within a couple of weeks, even if they continue to eat their own cultural foods. Their microbiomes lose a lot of species diversity so quickly. 
Conversely, you and I had the same exact experience living in China. We quickly went the other way and probably had healthier microbiomes while we were there. I always find it very easy to maintain a healthy weight while I’m there, among other benefits. 

Glyphosphate is just one of the underlying problems. It disrupts mineral uptake of plants as well as plant and human microbiomes.  For sure other industrial farming practices are involved, that’s just an easy one.