Two points:
1. There's slim (D&G, Dsquared) and there's SLIM (Dior Homme, Helmut Lang, old Raf Simons). The former tends to be tacky, gaudy, show-every-muscle&balls slim.

The latter is classy slim - I like to think of it as French chic slim (for non-Dior references, see APC catalogue [although it's arguable that brand is slim fitting, or Tati's films especially Playtime -

those pants)
2. Other brands may claim to be slim but searching for size S is like looking for needles in a haystack, therein lies the concept of opportunity costs.
I'm surprise no brand caters to the slim market before Dior Homme (Prada, HL and Raf Simons are the closest). Look who is laughing all the way to the bank now.