I think this movie is a spinoff\remake of another film. I've been looking for it for a long time. It also features a giant and many smaller killer rats but the special effects are far worse and the acting in this film deserves an oscar in comparison.
Does anyone know which one I'm talking about? Still can't find it..
http://www.imdb.com/title/tt0067991/
Also, there was a sequel: http://www.imdb.com/title/tt0068264/
You might be thinking of "Rodentz" (AKA "Altered Species"):
http://www.imdb.com/title/tt0292429/
At least this was the one I was immediately reminded of, when I first saw that huge rat in "Rats". I don't think there's any real, intentional connection between the two movies, but story-wise, "Rats" really could serve as a direct sequel to the events in "Rodentz". As you said, though, "Rats" is by far the better movie.