I'm dubious that most small presses sell fewer copies of each book.
You really don't understand the publishing business, do you?
But readers don't care whether the pubber is small or large or in-between. They only care if they like the book.
You really don't understand how most readers obtain the books they read, do you? In order to "like" the book (and remember that verb now has Facebook residue dripping off it), you have to
find the book. The vast majority of the book-buying public is really really lazy, or harried, or whatever, but just can't take the time to
search for books. Most people either go directly to Amazon, where sales figures etc. matter, or visit the big racks in bookstores or grocery stores, where the BIG BESTSELLERS are displayed with author names twice the size of title names, in embossed metallic letters. And most are looking for, or attracted by, names like James Patterson, Dean Koontz, Stephen King, Janis Evanovich, Faye Kellerman . . .
You really think those readers are going to become enamored of a book published by a small press and to be found only by making a concerted effort to browse and search?
There are a lot of respectable small presses that publish respectable books that the BIG GUYS won't bother with, UNLESS some form of lightning strikes, like having the President of the United States, or Oprah Winfrey, praise the thing to high heaven in a public forum. But I don't think anybody associated with those small press publishers makes a big amount of money, and that includes the authors they publish. Their business model is geared toward smaller avenues of gratification and success.
And don't bring up J.K. Rowling as a template for how other writers can hit the big time. Her story is about the most anomalous outlier on the bell-curve of writing success that I know of.
You know where most people buy books from small presses in my town?
The big used bookstore thriving here. I'm often among buyers there. They have lots of wonderful books no longer in press. No longer available anywhere else. And which generate exactly zero dollars for the author.
caw