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Most jeans are made of around 10-12oz denim, while Momotaro mainly uses 15.7 and 18oz denim. This means they are much thicker and more sturdy than your standard pair. This also means that when new, they are very stiff, almost cardboard like. They take a while to "break in", a process that some, including me, very much enjoy. Some brands like Iron Heart make jeans as thick as 25 oz. They are basically denim sweatpants at that point.
But the main things that separates Momotaro and other Japanese brands from your bog standard run-of-the-mill jeans are the factors of using thicker, higher quality raw materials, attention to details such as the filled belt loops and the hidden pocket rivets, and the sewing work being of a very high quality. It's just a pair of jeans made to a very high standard, nothing magical really.