I've been blogging for many years now for a variety of websites and also done a significant amount of ghost writing. The word counts for these blogs have range from a couple hundred words to over a thousand.
I often ask myself what is the ideal word length for a blog? If you go too short, then you risk not putting in enough information and not having a complete thought. If you go too long, then the reader gets bored and doesn't even bother reading it at all or reads only part of it and moves on.
In my humble, but experienced, opinion, about 400-500 words it the ideal blog length for your standard daily blog. If you examine many blogs, they follow this same thinking and limit their blog posts. People don't want to read War and Peace in blog form, but at the same time most people are not so ADD that they want more than just a couple hundred words.
Of course, there are caveats to every rule. For example, many sponsored posts require a certain word length or keyword density. It's not always easily natural putting a keyword into a post, so you end up writing longer because you literally have to craft places to put those words.
Contests also tend to run long because not only do you require information about the contest, but you have to jump through several hoops that the company providing the item for the contest requested. It could be a review of the product, information about the company, etc.