Some people call this the first SciFi book ever written (in about 1816), and I found out I'm really into 19th century tone of narration.
Yet the plot is not so large that the book should need 200 pages to get through. There are many passages with tedious self-reflection of both Victor Frankenstein and his unnamed "monster". Also, the plot isn't really good - the monster behaves like a gentleman most of the times, he learns exceptional english by listening to a French family etc.
But the "monster" isn't really a monster, it's just made to be one by Victor, his creator who does not fill the role of dad he also has in relation to his creation. The monster kills, but only once enraged and made to be forlorn by Victor himself, and all other humans, who get enraged just by seeing him.
It appears also that Shelley was made to turn this into a novel by her husband, Piercey b. Shelley, and this might explain the unneeded length.
I was reading the 1962 reprint of the 1934 Heritage Press book with illustrations by Everett Henry and an introduction by Edmund Lester Pearson, which is a fantastic issue.