I take commuting a bit to the extreme....about 30 miles each direction and live in the southeast where the weather is unpredictable.  I utilize frame bags for essentials that I may need quick access to.  Wallet, office keys, phone, and a few other necessities but I like riding with a backpack for work clothes and food.  The frame bag helps keep my backpack from getting too full, especially in the winter with so many layers, and allows me to run errands on the way home.  I use an older version of the deuter trail pack that has a built in rain cover for unexpected wet rides.  The bag has a rigid mesh panel that sits against my back and sits away from the main comparment for exceptional air flow.

With all this being said you can definitely find some frame bags that will fill the space inside your frame's triangle and can hold a lot of items.  I'd recommend something waterproof and possibly invest in a handlebar bag for the items you need to get to quickly.