WWE rarely even sells out RAW anymore, and they usually don't even run the 18,000+ seat arenas like Staples these days. If that's your standard for major league, there isn't a major league promotion in existence. According to Dave Meltzer WWE hasn't sold out a RAW event (even though they announce them as sellouts anyway) since late August at Barclays Center. WWE shows hit around 2,500 on the low end and 20,000 on the high end (except Mania, of course), but most shows are between 4,000 and 9,000. And NXT events sell out quickly, but NXT also bleeds money.
ROH gets roughly 1,000 fans per event. That's small potatoes compared to WWE but the majority of their shows are legitimately sold out with very few comps. Nobody else in the US besides WWE gets close to that. Not even TNA or Lucha Underground, let alone the real minor promotions like PWG or Chikara. Not even ECW hit those numbers until late 1998. Most indie promotions average in the low hundreds.
ROH is also on weekly TV (just the Sinclair station ROH broadcasts average 300K-500K viewers, not counting Comet TV, NESN, or ROHwrestling.com viewers), they're on cable PPV and iPPV, they tour most of the country these days, etc. To me, that's the mark of a major league promotion. They're available nationwide and they fill up the arenas they run in.
ROH is the biggest "pure" wrestling promotion in the US right now. That's the reality of it. They're bigger than TNA and they're bigger than Lucha Underground (LU is more of a TV show like WWE anyway). They're the biggest company in the US that offers the kind of product they offer. The market for "pure" wrestling is smaller than it used to be, but ROH is one of the only wrestling companies in the world that is actually growing right now. TNA, LU, and WWE are all contracting.
EDIT:
ROH stopped being an independent promotion when they were bought by a billion dollar broadcasting conglomerate. It's not a question of size, but of who owns them. They were an indie until Sinclair bought them.
