Steel Series makes fairly small gaming mice.
For example, SteelSeries Kinzu or Sensei are fairly small, with Sensei being just a bit bigger.
Sensei has the side buttons
Both have an easy sensitivity change button, which you can set from their driver engine: SteelSeries Engine.
With a push a button, you can quickly change to lower or higher dpi.
That is very useful on fps games. For when you need to do precise shots, you can change dpi to be lower and then change quickly back to normal.