I have 2 theories as to why in Indian and Chinese cuisine, our dessert course is lacking behind that of Westerners.

1. our starters and main courses are more filling, so we are stuffed by the time dessert arrives

2. we have less 'room' in our stomach

any other suggestions?

My suggestion is that your theory is ludicrous. There are plenty of Asian desserts, they are just not overly popular with the western palate, as a lot of them, especially Indian desserts such a gulab jaman, rasmalai, halwa etc are very very sweet, to the point of being incredibly sickly.

I can't stand Asian and Middle Eastern desserts, most of them to me are awful. I think the English, French and Italians are masters of desserts. Asians are of course masters of other things, notably spicy cuisine, rice and noodle-based dishes.