I carry a pretty large military style sleeping bag + pad. It is bulkier than I'd have preferred ideally but warm enough for temperatures down to about freezing. I should point out though that I never sleep in a tent so I need a slightly warmer sleeping bag.

A friend of mine just brings along two light sleeping bags when the weather gets colder. Seems to work really well putting one bag inside the other because of the extra insulating layer of air trapped in between.