What has this got to do with bubble bath?
You may have noticed that foam doesn't last forever. One reason is that the bubbles eventually burst, but there is another effect which encourages this and is connected to this experiment. Bubbles are blobs of gas surrounded by thin layers of water. This water has surface tension which is trying to make it as small as possible, a bit like the rubber in the balloon, but water has an even more strange force extension curve. The force is a constant whatever the extension, So the only thing which affects the pressure inside a bubble is how big it is, a small bubble will be at a higher pressure than a larger one. This means that gas will tend to leak through the walls (by diffusion) from the small bubbles to the larger ones, making the small ones smaller and the large bubbles bigger. So over time you get fewer and fewer larger and larger bubbles.