Sometimes the publishing world changes rapidly. Things change from one state to another with hardly a step in between. In this case the early 80's are the dawn of the brick.
It's like someone woke up on January 1, 1981 and said "From now on, no books less than four hundred pages long! And five hundred or more would be even better." Before 1981 the average page count of the Hugo winners was roughly 250 pages. There was occasionally a monster book like Stand On Zanzibar but they were exceptions. Fountains of Paradise by my rough reckoning is the last winner under at that size, then suddenly everything doubles in page count. The Snow Queen and Downbelow Station top five hundred pages, Startide Rising approaches that number, and even Foundation's Edge is grasping at the four hundred mark.
Despite the added page count a lot of these books don't feel like they're really have anything more than the previous books. Cherryh managed to use the additional pages in Downbelow Station to pack in interlocking subplots but the other long winners felt like they could have been streamlined considerably. It isn't simply "too big to edit syndrome" (TBTES) the disease that effects authors who's books sell more than 100,000 copies and causes them to overwrite their books and refuse to cut out the useless sections (Stephen King and J.K. Rowling are famous victims of this debilitating illness); none of these authors were "too big" when they published these books.
My personal and completely unsupported theory is that printing improved in the 70's making better bindings more viable. Once that was in place it was possible to make bigger books more cheaply. Book buyers when presented with a book at 250 pages and another one at 500 pages at a close price chose the 500 page one (I'll admit at the time I was one of those; I have since learned about the pains of TBTES). Editors noted this and started looking for longer books. More books of that length published and read would equal more readers to vote for them in the Hugos and here we are.
Other than the books suddenly becoming huge there were a few notable things in this period. The Snow Queen was the first "fantasy" book to win. Admittedly Vinge tied it up in science fiction dressing but the style was all classic fantasy. Hugo voters still weren't really to go all out for fantasy yet and it is the last for a while.
Foundation's Edge is the only Hugo for novels that I would call a lifetime achievement award. The book was terrible but the award was for Asimov himself. This is not repeated.
We're starting to see the impact of the linked novels on science fiction in the awards this time as well. One winner was a direct sequel (Foundation's Edge), and two other winners were new novels that used existing settings (Downbelow Station, Startide Rising). Before the early 80's there were books that got sequels and one book that used an existing setting (Ringworld) but now we get a lot of books that are either sequels or written with a sequel in mind.
On the liked/didn't like scale:
Didn't Like: 11
This is the worst block of winners for me with three books I didn't like: Fountains of Paradise, Foundation's Edge, and Startide Rising. I enjoyed Downbelow Station and The Snow Queen which curiously enough also means that I liked all the books by women and didn't like the books by the men. Feel free to read something into that.