I ride with a honkin' big pannier (left one) all the time. Today I rode farther than I expected. Emptied my bottle. No prob, got another one in the pannier! Hungry? Banana, Lara bar, Payday, Clif blox, Clif shot, walnuts in the pannier! Front fender had a nervous breakdown. Whaddaya know, I have a Leatherman and Park Tool I-beam in... the pannier! And once I got the whole thing off I stuck it in the pannier! Starting to rain? Rain gear in the pannier.

And pump, tube, hankie, sunscreen, lip balm, ouch-pouch, etc.

I also have a bungee cord wrapped around my rack. Just in case. I'm a belt-n-suspenders kind of gal!

(hey, I might need to carry a 12 pack of beer, too!)