How much stuff can fit in a 30-40 liter backpack?

I'm really curious. I haven't started traveling just yet, but I'm still interested in traveling to places/different countries so I've been researching backpacks, what to bring, etc..and I've seen mixed advice...Some say that you would need a 60L or higher backpack to travel for several months, but other people (only a few), say that they've traveled for several months with just a 30 liter backpack, and it worked out just fine! They also said they packed lightly, but I wouldn't mind doing that; I'm just not sure what they meant by lightly! So how much can fit in a 30-40 liter backpack? Like how many sets of clothes, along with shoes, socks, underwear, and anything else that would be necessary? I'm mainly talking about for warm weather travel. But also, is it possible for a backpack that small to be used for winter travel?