Large print gets treated as a single feature, like a tick in a box. It isn't. A book can be set in 18-point type and still be tiring to read, and most of the reasons have nothing to do with the size of the letters.
Contrast does more than size
Black ink on bright white paper produces glare, especially under the ceiling lights in most homes and almost all care facilities. The page seems to shimmer slightly. People read for ten minutes and get a headache and conclude their eyes are worse than they are.
Cream paper solves most of that. The letters can even be slightly smaller and the page will still be easier, because the eye isn't fighting the background. It's the least glamorous decision in book production and one of the most useful.
Space is part of legibility
Line spacing, margin width and the gap between words all decide whether a reader can find the start of the next line without hunting for it. Cram large type into a normal layout and you get something worse than ordinary print: big letters, no room, and a line that's hard to track.
This is why a genuinely large-print book has fewer words on the page and therefore more pages. It costs more to make. When a book is advertised as large print and is the same length as the standard edition, the type got bigger and the spacing got squeezed, which is a wash.
Weight matters more than people expect
A book that's uncomfortable to hold gets read in shorter sittings, whatever the type looks like. Anyone reading with arthritic hands, or in bed, or in a chair with no table, is making a physical calculation every time.
It's also why a book that lies open without being held down is worth more than its specification suggests.
One thing per page
For puzzles, prompts and anything meant to be worked at rather than read straight through, the single biggest kindness is not putting two on a page.
Two puzzles per page saves paper and costs the reader their place. It means covering half the page with a hand. It means a person who has to stop after one has to remember where they were. One per page is more expensive and it is the difference between a book somebody finishes and a book somebody abandons on page nine.
Why it usually gets skipped
All of this — cream stock, generous spacing, one item per page — adds pages, and pages add cost. In a market competing on price, the first thing to go is the white space, because it looks like nothing. It is not nothing. It is most of what makes the book usable.
If you're buying for someone whose sight is going, the specification worth asking about isn't the point size. It's the paper, the spacing, and how much is on each page.