The Gum Cam is very popular because of its size. Even the name explains it well: so small that it can fit inside a pack of gum.
Typical dimensions are 2.874in x .7874in x .4331 in. This is a very tiny size. You do not necessarily have to put it inside a pack of gum however!
As you can see, the gum cam is very small yet packs a lot of stuff -- a microphone to record voice, a USB port to transfer video files to your computer, a slot for a micro SD card, and numerous other features.