Relatively speaking, HD (720 or 1080) uses 1-3 GB per hour and 10 GB per 24 hours would be about 450 MB per hour. 450 MB per hour is equivalent to streaming very low-quality video (240 or 320). I'm not saying this is the video format but the equivalent data rate of 10 GB per day.
I think your app points to a server on the Internet. When you use the app, you're not logging into the camera but into a D-Link server. Although you're not constantly recording, streaming or using the app, the camera(s) must send something to the server to keep your service (connection & app) "alive" 24/7.
I don't know what the camera is sending...perhaps an image every hour or an image after a massive change in pixels...but it's the cost of a web-based service.
widermouthopen is correct: You need to ask on the D-Link Forum to configure your camera to only transmit data when you're logged into the app...if possible.