I've noticed that I'm influenced by the size of a book when I'm deciding whether or not to buy it. Not the thickness but its height and width. The best place to see the difference is at a branch of WHSmith. They divide general fiction books (that is, those that aren't 'genre' novels) into two sections: fiction and "literary fiction". The books in the fiction section, your Jackie Collins and Jeffrey Archers, are small. In literary fiction, where Margaret Atwood and Ian McEwan hang out, the books are bigger.
I worked in a bookshop as a teenager and learned that these paperback formats are known as 'a', 'b' and 'c'. 'C format' books are usually the same size as hardbacks, though I see from the Publishers Association website that this isn't a precise term.
However, the other two are more clearly defined. The literary fiction size is B format, which the Publishers Association describes thus: "a format for paperbacks particularly favoured for non-fiction and literary fiction, normally of a trimmed size 198 x 126 mm (unsewn)."
The PA describes A format as the "format of mass market paperbacks, most commonly with a trimmed page size of 178 x 111 mm (unsewn)".
This has become a kind of visual shorthand for whether or not I can expect to like a book. If I'm browsing those 3-for-2 offers that are permanently available at the likes of Waterstones or Borders I don't even bother to look at the table with the A format books on it.
It's become a kind of snobbery. If I read about a book which sounds appealing and then discover in the store that it's an A format book I have second thoughts about buying it. Likewise if somebody recommends a novel, I feel slightly reassured if I discover that it's published in B format.
The distinction doesn't hold up so well in genre fiction - crime, sci-fi, fantasy etc. - almost everybody is in A format. In crime even the likes of Ian Rankin and PD James come in A. I can't think of a contemporary crime writer whose work comes in B format. Perhaps the snobbery works both ways and crime readers would be put off reading a B format book?
In sci-fi, Iain M Banks - the sci-fi alter ego of Iain Banks - is about the only one who gets B format. Even legends such as Asimov and Clarke have to make do with A.
Sometimes people make the step up. Sci-fi author Michael Marshall Smith, who now writes A format thrillers as Michael Marshall, had all his books upgraded to B format a few years back.
It's a way of signalling to snobs like me that the book is worth looking at. Why else are JK Rowling and Philip Pullman in B? You'll never sell an adult a kids book in A.