A discussion page is a page that has been designated as an appropriate place for ThreadMode conversation about some other page. Usually the name of the DiscussionPage is derived from the original page name with the word "Discussion" appended to it.
When in the process of converting Pages which started as documents, but have accumulated threads of discussion and q&a, into document-mode, a DiscussionPage, containing the thread-mode content may be created. While the document may be just a little more than a Single-Page, the DiscussionPage may accumulate stuff until for some, it is TooBigToEdit.
This may be rectified by persons who would use the LessIsMore concept to remove some of the less important discussion and extraneous stuff, thus making the page more readable and sensible.