http://www.transpack.co.uk/faq.asp?FaqID=3

Which size bubbles should I use?

This depends on what you are packing and how you want to use the bubble wrap. Most products can be easily and well packed in small bubble wrap. However, the large bubbles are better for delicate shapes such as wine goblets etc. The large bubble wrap is also better (and cheaper) as a void filling medium.

What's the difference between large and small bubbles?

Well, to start with see the answer above. However, the fundamental difference is one of softness. Large bubbles are softer and thus more suited to delicate things, unless you just need the extra thickness to bulk out a box.

But, small bubbles will carry more weight. For example, if you wrapped three heavy doors in large bubblewrap you may find the bubbles bursting with the weight. In such a situation smaller bubbles will do better.