CO2 Inflators

The kind that takes threaded co2 cartridges are usually a lot smaller since they don't have to encase the entire cartridge to puncture the co2. 

I use an xlab co2 chuck that accepts threaded cartridges and is small enough that I can wrap it in a spare inner tube and stuff both under my seat. 

I've got one that will do threaded or unthreaded. So I can buy the cheap CO2 from walmart, or in a pinch use someone else's or grab some at a bike shop.

And I always carry 2 cartridges.

I use this:

http://www.genuineinnovations.com/bicycle/commuter-touring/second-wind-road-carbon.html

I'd consider this a good solution for regular training rides, but not racing.  It's a lightweight pump that also accepts CO2.  So, if you have more flats than you have CO2 cartridges, you can still get home. Granted- it would take a long time to pump up your tires with it, but it's handy for partially inflating your tube to install it, and putting in a little more air to check seating.

On the downside- it rattles a little over rough roads and takes an extra 30 sec or so to strap and unstrap it from your bike, and then screw in the CO2.  For race day, I'd want something already set up and ready to go.  Consider an air chuck, which is just one of those little ends that screws onto the threaded cartridges.