Small presses often just don't have the budget to market their authors, but not so with the big houses. They have a vested interest in maximising sales; having already made a huge investment in advances, printing, artwork, distribution etc it would be pretty dumb not to.

And I would imagine, it's true, even for large houses. They would do some marketing (such as reviews, some publicity and book signings, etc.) but budget is tight. The money goes to the big names, leaving little to the lesser-known authors. It's up to the authors to market themselves and the books—doing signings, readings, putting together a website, going to conferences, and talking about their books every chance they have.

Of course, I only have indirect experience with the larger houses!