As ultra-light bike tourists go I'm not that "ultra", but the image above is a week's worth of gear including camping, cooking and purifying water. I tour in Canada so summer still requires a cold weather gear and that's in there as well as rain gear.

Forget that it's a mountain bike. You can use the same approach on a road bike or cyclocross bike.

If you are staying in hostels or couch surfing and eating cold meals or buying prepared foods you can drop a bunch of stuff I have in those bags.

My transition from 4 panniers + handlebar bag + stuff on top of my racks to this took some time, but the most effective thing I did was buy small bags and force myself to only take what I could fit inside.

Even when camping deep in the backcountry I don't wish I had more stuff with me.