They use thinner boxes because it's rare that they do international shipping and don't need heavy duty boxes. They can ship everything ground and have it in 3 days or so for the cheapest option.
A buddy of mine worked part-time at a post office in Korea for his summer job. This is what he told me when I asked about what's different compared to what I told him about the US. Might be different in Japan, and I know it's different in China.