Hi Bobby,
Can't really tell from your pics if they are overweight/underweight. For Shih Tzu with a good coat it can also sometimes be difficult to say by looking at them - you really have to feel them. Well, as long as one is not grossly obese and the other not emaciated, then they should be OK.
Why are they different like that even with the same amount of food? Well.....it's kind of like why Roy and I are putting on the pounds whereas Edwin is skinny as a stick even when he eats more that the 2 of us combined!

Maybe just genetics and metabolism.....
Anyway as long as the are active, eating well and otherwise normal, just KIV and don't let the fatter one get fatter!
Interesting options to stay in Thailand...I know that there was one scheme where you had to invest a certain amount (US$200,00??? - I forget) for a retirement visa or something - one of Thaksin's schemes I believe. Is that still on? As for Roy....he's actually thinking hard about something similar to yours...

Haven't been up to Chiang Mai in many many years.....maybe it's about time to visit - I understand there will be a large garden show from the end of the year to Feb next year.....