I'm not sure that's a fair criticism
. Most of Hardy's books are a lot shorter than Dickens or Eliot! "The Major of Casterbridge", for example, is about a third the length of Eliot's "Middlemarch" or Dickens' "Bleak House".
The main thing about Hardy is that he's just so incredibly depressing to read
. He was the first English novelist to "escape" from the custom of having comedic (ie "happy") endings to books, and use tragic ones. In most of Hardy's books, the main character ends up ruined, committing suicide, executed, or some other such cheerful fate!