Zaras are not slim by European standards. I would rate them to be slightly more generous than RL custom fit shirts. Also Zara shirts come in two grades, the standard and a higher end one with pearl buttons. Price difference about 30% I think.
It depends on how the shirts are labeled. The shirts that are labeled Size S fit me almost perfectly, while the ones that are labeled size 38 are quite a bit blousier in the chest and waist and fit more like RL custom fits. I can't afford designer stuff like Dries Van Noten or Margiela yet, so Zara will do for now