Size of event is not determined by difficulty/complexity of tournament format but by the strength of the competition there (in other words, rankings of the attending teams). Agree or disagree, I believe this is how HLTV judges the "size" of an event.
Dreamhack Open X (Insert random city) can have the most intricate playoff system ever but if there are no Top 10 teams attending, then it is not a big/Premier event.
For example, I'm sure CS:GO Asia Championships would have been considered a big event if there were more Tier 1 (Top 5-7, I guess?) teams attending.
P.S. Before you start calling me names and my comment invalid, please understand this is what I believe to be HLTV's criterion for what's considered a Big Event. Feel free to have your own but ultimately, only the official (HLTV's) one matters.