Its attitude, education and language. The US, Australians, british ,europeans have similar culture and education so they look at each other for inspiration.
The difference in leg sizes are not as drastic as you are making them to be, and even if they were it should not matter sf soldiers. They are the best the Indian army has and are very few in numbers, custom kit should not be a problem.
A summery on what the US guys use in cold weather. Its crazy how much thought they put into it. I would love a video like this on what we use in Siachen.