I'm really curious to know how much effect the size of a publishing house (generally) has on the sales of a book. I assume that the bigger houses have more money to spend on promotion, but do they also get the books into more stores? I find that when I browse at Chapters, the majority of the books on the shelves are from big houses. Do the smaller houses have a hard time getting shelf space?
yes and no.
first, you're confusing promotion, marketing, and "taking a position".
Promotion is all the stuff that goes into making you aware of the book outside the store: radio interviews, buzz, mention on blogs, Miss Snark accosting you at your door with the good news that it's time to buy one of her books.
Marketing is the advertising, outreach to stores, things generally that are paid for by publishers. Included in this is "new voices at GotRoxStore" "new non fiction" "hot reads" etc.; those big displays you see on the front table of chain stores; and all the swag you get like bookmarks, magnets and "car with purchase" kind of thing. Yes, publishers pay bookstores for placement. Yes it happens all the time. No, they don't talk about it much, and bookstores scream with outrage when the word "payola" is bandied about but it doesn't make it any the less true.
So, yes, smaller publishers with fewer swag dollars and payola accounts have less muscle when it comes to that stuff.
Taking a position means big chain stores will get behind a book..or not. Large publishers can have books that tank just like small ones. If the head fiction buyer of Barnes and Noble, Sessalee Hensley takes a big position on "Miss Snark's Guide to Etiquette" it's going to be displayed big. Publishers spend a lot of time reading Miss Hensley's tea leaves and trying to persuade her to do just that.
However, that does not mean you should prefer a big publisher to a small one. Small publishers have to be nimble and they know it. They're more likely to spend less on shotgun approaches and more on target approaches cause they have to.
And lastly, it's not the size of the publisher that you really want to investigate. It's how they get their books into the marketplace. Whether they have a distributor is key. Small presses who are distributed by Consortium are MUCH better places to be than publishers who only sell books off their website, or who say "available through Ingram".
"Available through Baker and Taylor, and Ingram" means you can order a book there, NOT that a bookstore will stock it. Many of the large publishers also distribute (ie handle the fulfillment and sales to bookstores) for smaller presses. It's how they make a lot of their money.
And none of this has a direct correalation with how books sell. About 75% of the dollars that roll into publisher's coffers come from the back list...books published last season, last year, or the last century. They're not what's "new! and ! exciting!" but they are what keep the ConEd bill paid.