1. using thicker paper
2. using a larger font
4. more spaces between lines
5. larger margins
6. a bunch of blank and/or pointless pages at the beginning
7. ad pages at the end
I see it all the time. A book's contents can vary 2:1 in length compared to another one of the same physical size.
I have the above analogy from teaching my kids not to be afraid of long books. It actually works both ways. "Look you finished hp6 when you were 12, got is like the same size don't be afraid."
Each of AGOT #3 and #5 on their own have almost has as many words as the entire LOTR trilogy. And while there are a lot of memes about how half of those words are detailed descriptions[1] of incredibly elaborate meals, I wouldn't say that they detract from the story.
[1] Onion grease dribbling down double-chins as wedding guests are gorging themselves on large pies, wide as wagon wheels, stuffed almost to bursting with carrots, onions, turnips, parsnips, mushrooms, and chunks of seasoned pork swimming in a savory brown gravy.
¹ https://en.wikipedia.org/wiki/A_Song_of_Ice_and_Fire#Publish...
² https://www.anycount.com/word-count-of-books/harry-potter-wo...
It's not long enough, so he does almost all those things you mention. It doesn't end well for him.