Developing my flipbooks

While studying the fundamental principles of animation we were given the opportunity to put these acquired skills to some practical use by creating a flip book. For my flip book I animated two bouncing balls which combine to form a single bouncing ball when they collide. The GIF above is played back at 12fps, animated on twos at 24fps, as is the standard for Animation. 

The main principle I took into account was the principle of squash and stretch, which gives the ball volume and weight to it's motion. One problem I have however is that when the two balls combine the volume does not change to a large enough extent, taking away from the authenticity of the motion. Maintaining the volume of objects in my animations is an area I need to improve in the future.