Hi there! I used to have the traveller backpack in XS then "exchanged" it for S. I find the XS is too small, although it fits the min necessities (phone, keys, small wallet, lipstick..), but nothing more. The S size is much more practical and it is not bulky at all, it can fit a 0,5L water bottle for example or a thin cardigan, some other bits. This is of course concerning the women's traveller backpack, because the men's one is super huge for a lady

Also depending what height you are. I'm personally medium height.