It is a fact that slams are big tournaments. It is not a fact that Olympics and WTF are big tournaments.
Actually, no. All is a matter of opinion.
Slams are big tournaments, but so are the Masters and Olympics.
So are the M1000's, too, for that matter. Slams just happen to be bigger. And that was no necessarily always the case (see the AO in the 70's and most of the 80's, it was then less important than the Masters and Dallas WTC finals).
So, everything is a matter of opinion.
Of course, you might also decide that only the bigger stages count. But in this case, what you wrote shows you're biased. If you only aim for the very top of the line in tennis, then there is only one single tournament, not four. And that is Wimbledon. All the others are smaller, and I mean all of them. Does that mean they're not
big, as you say?