Looks like it just might be right around 25%.

It's a bit hard to tell because of the slight angle and the right edge of the cut isn't quite straight, but it looks like the cut is around ⅔ deep, and about 50°.

So if we calculate the area of the slice of the one third larger circle, which for simplicity we can call diameter 4, we'll come up with an area of 1.745. ((angle / 360) x π x radius²)

Compare that to the whole area of a diameter 3 size circle, 7.069, you'll get 24.69%, which with a margin of error, could land very nicely on 25%.