I cannot give a definitive answer, but looking at them is seems so.
If you go your book shelf and pick out a random book, you will most likely find that the bottom and our margins are the biggest.
If I recall correct, the reason behind this is that these places are where you are holding when you are keeping the book open for reading and when you turn pages. In older, especially handwritten, books, any text there would be prone to smearing of the ink, thus they made sure to have sufficient space for your thumbs.