Many people don't realize that when they install an app onto their iDevice that by default, it's data is added to their iCloud backup. I didn't realize this until very recently and I've had an iPhone since November 2012!
Every iDevice user gets 5 GB of free iCloud storage from Apple. This is not 5 GB per iDevice but 5 GB per Apple ID. If you have an iPad and an iPhone, like I do, where you are using the same Apple ID, the cloud storage is shared. The problem with this default backup setting, is that unbeknownst to you, your storage could be eaten up with app data that doesn't need to be backed up.
The data from Evernote, Kindle, your social media apps (Facebook, Twitter, etc.), and Maps—to name just four—is already all in the cloud and does not need to be backed up to your iCloud too.
To see what is actually being backed up to your iCloud, go to Settings > General > Usage. On this screen, you will see Storage along with the amount of space you have available on your iDevice (not your iCloud) and the space you have used. Here, you will also see every app you have installed along with the amount of space they are taking on your iDevice.
Scroll to the bottom of this screen and click on Manage Storage (right above Battery Usage). On this screen, you can see all your Backups along with the total size of the backup, the size of each iDevice backup (see below), Documents & Data and Mail.
At the very bottom of the screen, you will see x.x GB available of y.y GB on iCloud (mine says 4.3 GB of 5.0 GB).
From this screen, choose which iDevice you are currently using. Under my Manage Storage, I see Mark's iPhone and Mark's iPad. If you only have one iDevice with this Apple ID, you will only see one device listed.
Next, click on the Backup.
On the next screen, you will see Your iDevice, when your latest backup was, and the Backup Size.
Below this, under Backup Options, you will see every app that is installed on your iDevice. Note: you may have to click on "Show All Apps" to see, well, all apps.
What you back up to the iCloud is up to you. For me, the only apps that I have turned on are Pages, Numbers, iMove, and Find iPhone. The other apps' data is either already in the cloud (online) or the apps don't have any data to be backed up.
Another note: back on your Manage Storage screen, make sure that the only iDevices listed here are active iDevices. When I first looked at this, I saw three iDevices and my backup was at 4.5 GB. I had given my wife my 4S (which she signed into with her own Apple ID) but I never deleted the backup. It was over 1.1 GB.