By moving the dot under the α=30, you can change the size of the angle. By moving the dot on the lower right vertex of the triangle, you can change the size of the triangle without affecting the angles. Notice that as long as the angles stay constant, the value of tan α does not change regardless of how big you make the triangle.
http://teachers.henrico.k12.va.us/math/igo/07RightTriangles/7-4Trigonometry/7_4tangent.html