The type wasn’t reset when paperback books got larger. The old typeset film was blown up to fit on larger pages. The print became less sharp than a typeface reproduced at its intended size. The lines of each letter are less precise than we once believed. The shapes of the different letters indistinct.
Because they can read the space around the print more clearly, modern readers are now able to find the true meaning of the words on the pages more than they’ve ever been able to before.