This is one of those things I only really noticed after seeing a stack of paperbacks that just looked… off. The US mass-market paperback is practically a cultural icon at this point—that compact 4.25 x 6.87 inches size. It fits perfectly in a back pocket or a purse, and the paper feels almost newsprint-y. That’s the standard for genre fiction, especially romance and thrillers, where you're meant to plow through them. Then you have trade paperbacks, which are all over the map, but often around 5.5 x 8.5 or 6 x 9. That's your literary fiction, your book club picks. Hardcovers tend to mirror the trade paperback dimensions before trimming, so they feel more substantial.
What’s funny is how much you can judge a book by its trim size before even reading the blurb. A tiny mass-market tells you it’s probably a fast-paced, plot-driven thing. A tall, slim trade paperback often signals ‘serious novel.’ I’ve got a few imports from the UK that are a different trim entirely—slightly taller and narrower than the US equivalents, which always makes my shelves look a bit chaotic. The actual reading experience changes, too; a bigger page with more whitespace feels more leisurely, while the cramped mass-market pages make you read faster, I swear.