A PhD thesis is a written scientific work between 9 [source] and 2000 [source] pages. The average page count varies for different scientific fields as depicted in this nice diagram from Robert T. Gonzalez Blog
The question what a phd is, should be of interest for a phd student. If you cannot find or formulate your definition of a phd, then it will be hard for you to perform well as a phd student. How can you do something if you do not know what you are doing or for what you are doing it for?
You have to find your definition of a PhD thesis. A search leads very fast to 2 great blogs about what a PhD is and what the difficulties in academia are:
A must see talk of James Hayton is http://youtu.be/4MkRMp3roKQ . He’s not only explaining how you do not go insane, he also provides you with the big picture. Another must see is The Illustrated Guide to a Ph.D. of Matt Might.
I hope you find some of the provided information and links helpful for your decisions.