Type 1 & Type 2 have to be viewed completely differently, but very often get conflated in these types of discussions. That’s also where your comment about whether diabetics are thin or not relates – many of the T1 diabetics I know are thin, many of the T2 are not. T2 used to be called “adult onset” until so many children started getting diagnosed with it.
For Type 1, there’s genetics (including insulin production and resistance, and fat ratios and locations) first. For Type 2, behavioral and environmental factors.
You mentioned rice and other carbs as a possible cause. Given your familiarity with Indian food, you know that South India is a heavy rice consumer. However much of the rice eaten there is par-boiled. Much other carb consumption is after fermentation (idlis, dosa, etc).
Science is only recently catching up with how those things impact insulin response — for eg par-boiling converts some of the starch to resistant starch, which dampens insulin response. (See this other thread about sequencing, combinations, etc as well.)
Causality is very complicated, and often not what it appears on the surface.
