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Mass and Shrinking.
Mass was just a function of one's impact on the space around them, and by extension, gravity. Mass was constant while weight fluctuated depending on the g forces acting on a body. How much a Mass warped space decided its gravity.
Speed impacted Mass as well. As speed approached that of light, Mass approached infinity.
The faster one wanted to go, the more pressures weighed one down. This was why it usually required breaking laws of reality, or rather having the strength to, that allowed one to break these kind of rules and exhibit strength and speed beyond reason.
However, there were other methods as well.
What Ryu had realized, however, was that he could control space such that his mass had a smaller and smaller effect on it. At the same time, his Northern Heavenly Wind also had the capability of making him lighter and lighter. By stacking these effects, he could limit the heaviness of his body and his higher and higher speeds.