Every time you send a request for a website or send an email you are uploading data.
If your upload data is very excessive, I would suspect that your devices have been corrupted with spam ware or malware. There are also many sites that you might be visiting that are sending information back to their servers gathering your browsing habits, and then data being collected by your operating system. Much of this can be controlled by setting the proper limits in your browsers and OS.
Are you using any apps that use the cloud? Those will use large amounts of data uploading and downloading. Tablets and iPads use the cloud for most of their operations, since they have limited onboard storage capabilities.