Fiery Keeper of the Hat
That advantage has some modulation to it. If I'm with a traditional publisher, then in my ad I get to say I've been published by Orbit or Tor. That's going to make that ad more effective than a similar one from a self-pub. In addition, the publisher themselves are doing at least a modicum of promotion. If nothing else, my book appears on their company site. So there are more points of marketing contact than I have if I'm self-pubbed.
So any of the traditional big publishers can be expected to do two things to promote a book: Send it to a list of reviewers, and spend 30 seconds pitching it to the buyers at bookstores. They will give a handful of top sellers a promotional tour. The only way they spend on ads is if your book ends up in like Target or Walmart, which is mostly top selling romance and thrillers.