Not Sveta, but often times where traveling multiple cities, you are using either trains or buses from one destination to the next. You cannot check your bag in either one. Also, you need to handle your bag yourself, which often includes raising it above your head to fit it in overhead luggage racks. The bigger the bag, the more you will pack, the heavier it would be. If you are just going to one city and back, I say travel a huge as you would like. A bigger bag is also harder to fit in European cabs and cars, their cars are generally much smaller and with much less trunk space.
Also, not that it matters, but people travel really light in Europe, much more lighter than my version of light. I did not want to be the one with the mutiple bags or the big ole bag, holding everyone up.
