An interesting question. Having a big bag on my bike is handy because I can throw all sorts of stuff, like a rain jacket, into it while I'm on a ride. The problem comes in cleaning it out periodically. One spring I found a dried up banana left over from the previous fall.

The flip side of the coin is the amount of tools and spares that some posters claim to carry with them on every ride. I've been criticized several times for saying that I only carry enough stuff to handle one puncture.