That's because it comes with a load of built in functionality.

I am sure Flasks documentation is a lot smaller but its essentially just a few parts of Django (it doesn't have a built in ORM from what I understand). Start adding in the documentation for SQL alchemy, documentation for an authentication library and whatever else you need and you will probably start getting something similar but without the guarantee that the pieces will play nicely together.

(Djangos documentation is really good.)

Having extensive docs is a very good thing, much better than having a short readme and then rely on stack overflow to eventually fill in the gaping holes.

It might be a lot of pages, but Django has some of the best documentation out there. Speaking as someone who had to recently use it for a project for the first time, it was very easy for me to find the information I needed in their docs.

Why is that a bad thing?

It's got wide feature coverage and excellent documentation.

I think maybe you're insinuating it's overly complex or hard to learn in which case come out and say that so we can engage with a specific assertion.

Isn't there a way to abstract that, using some of Django's libraries ?