Thank you very much. The confusion came from my belief that Kindle somehow "rescans" the book every time we change font size which causes a book of actual lenght, say, 250 pages become 125 pages when I select a tiny font or a 1000 one if I select a big one instead of just referring with the page numbers to the "real" book
That's how iPad works with the books (iBooks app, specifically) and as I'd been using it as my reading device before getting a Kindle, I considered it a "proper" way to do things. Seeing my Kindle doesn't do such thing led me to the conclusion that something had to be broken.