Why does it happen?
If you have ever tried blowing up a balloon you will probably have noticed that to start blowing it up you need a huge pressure but once it is started it becomes quite easy until it is fully inflated when it gets hard again. So when balloons are just starting to inflate easily, a smaller balloon will be harder to blow up (at a higher pressure) than a slightly larger one. So if they are connected air will flow from the small to the large balloon increasing the difference in size.