How to Travel Light - No Seriously, How Do You Do It?

I have been toying with the idea of bringing just a carry-on suitcase on a 21-day trip this July. I know, I know, it's still a ways away, but I am already dreading the thought of lugging around a full-sized suitcase along cobblestone streets, and hauling it up and down the stairs. 

Many people have written about how it is possible to live out of a carry-on suitcase for three weeks, a month even. I read a lot of these blog posts and every single one of them shows a picture of an overstuffed backpack or small suitcase, with no room to spare if you want to do a little shopping. Also, do these people not know that the same number of clothes take up more space once worn? It's like you can never fold them the same way. Ever. 

So, what gives? Do these people not shop at all? Do they discard pieces of clothing before they go back to their respective countries? 

I once traveled to Singapore with a couple of friends from college. One of them brought extremely old clothes to be discarded on the last day. But old clothes that you're ready to give away? I would like to look good in my photos. 

I guess bringing just a carry-on suitcase is a bit too extreme for me. Even if I know I will never have the money to do a lot of shopping, I will still need extra room for a few souvenirs here and there. So for now, I have decided to bring a medium-sized suitcase - big enough for all my clothes and with room to spare for souvenirs. By the souvenirs I mean food.