People overstate how small Marner is. You guys realize that he is 5'11 and 170 lbs right? If you watch him on the ice he looks like it too- small but definitely bigger than players like Konecny (5'10, player that I can think of off the top of my head).
Just looking at the top 25 scorers in the NHL right now I can list several that are the same height and/or weight as this guy, with their scoring rank listed:
1) Patrick Kane (5'11, 177)
6) Sidney Crosby (5'11, 200)
8) Artemi Panarin (5'11, 170)
9) Joe Pavelski (5'11, 190)
17) Nikita Kucherov (5'11, 178)
23) Claude Giroux (5'11, 172)
Mitch Marner listed at (5'11, 170)
And if you include top 30 scorers then add Duchene (5'11, 200) and Steen (5'11, 212).
If people are comparing him to Gaudreau (5'9, 157) and MSL (5'8, 180), then I feel you could also compare him to Seguin, O'Reilly and Bergeron (all 6'1, 195-210). The difference in height at least is in the same level. He is a small player but definitely not in the realm of tiny.
Edit: It could be argued that he needs to get stronger/heavier and for that I agree, but look at the all the players I listed above, all older than Mitch by a few years- he will get stronger over the years (he's only 18).