I have a fairly small seat bag that I keep tire fixing stuff, id, $, and my koffee caps.
On a recent ride with my friend he proudly showed me a tiny, tiny seat bag he'd just bought implying it was lightweight and only big enough very minimum amount of stuff.
When we ended the ride I had to laugh as he pulled way too much stuff out of his jersey pockets, ya' know, much stuff he could've stuffed in an bit larger seat bag.