As the triangle swings through the top-right quarter of the circle, the horizontal leg is the adjacent side and the vertical leg is the opposite side.
cos(theta) tells you how much of the radius survives horizontally. sin(theta) tells you how much of the radius survives vertically.
When the point nears the right edge, cosine is close to 1 and sine is close to 0. As the point climbs upward, cosine shrinks and sine grows.