How does Windows 7 Backup & Restore go about backing up files?

My backup file is written to separate 300 GB drive. Over time, the file grows to the point where the drive it's on runs out of space. Well, I don't have 300 GB's worth of new data at any given point, so how is this happening? It should only back up files that have been added or files that have changed, but, if it did that, I wouldn't constantly be having to clean up my old backups. In fact, I don't even know why I have to clean those up. There also should be no such thing as an "old backup". There should be only one backup and it should simply be "up to date". The only thing I can think of is that WBA is creating a file history which means, if a file changes, it backs it up again without deleting the old.

Edit: Added thought: When I do delete old backups, what am I deleting? Does an old backup contain anything that a new backup doesn't? Is there any way to get WBA to automatically clean up old backups X number of days old?

Is there a document somewhere that explains what, exactly, WBA is doing behind the scenese?

Edit 2: When anwering, please try to address my specific questions. I will outline them below for your convenience:

• Does WBA backup the same files over and over again rather than just updating existing ones?
• Why does my backup file size accumulate when I am not adding anywhere near that amount of new data?
• What is in an old backup?
• Does an old backup contain anything that a new backup doesn't?
• Can WBA be configured to automatically delete backups that are X days old?

Does WBA backup the same files over and over again rather than just updating existing ones?

Yes, each update contains whole files. If one always updates the same files, then these same files are saved again and again.

Why does my backup file size accumulate when I am not adding anywhere near that amount of new data?

Because the same files are backuped again and again.

What is in an old backup?

A backup contains files and a directory that lists their placement.

Does an old backup contain anything that a new backup doesn't?

Not necessarily. An incremental backup may possibly only contain files that are superseded in newer updates, but there is no way of knowing that.

Can WBA be configured to automatically delete backups that are X days old?

Only an entire backup set can be deleted, as one can never know which incremental update is useful (meaning contains a file that is found nowhere else).

Windows 7 Backup has a feature which allows you to manage backup disk space. On the Backup and Restore Center page, look for the "Manage Space" link. You will find it right under the target description.

The following blog post walks you through the UI and the space management options available: Managing backup disk space.

Windows Backup takes full backups of data from time to time. If you want for example to set up full weekly backups, this registry entry is said to tweak full backup frequency:

Path: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsBackup\AutomaticFullBackup
Name: TimePeriodInDays
Type: DWORD
Value: the default value is 365. Set it to 7 for weekly full backup.

However, there are reports that this may not work as expected.

I remark that there have been many problems reported with Windows Backup on this forum. There are too many gotchas and one risks finding that the backup taken is unusable at the worst possible time.

I really suggest using instead a third-party solution, of which there are many, as being much safer and easier to use. See Best Free Backup Program (including the comments). (Edit: An archived version of this webpage is found here.)

Windows Backup incrementally backs up a lot of files that aren't significant, some of them very large, as they change even if nothing significant to me has been done. After the second backup, and every so often, I open the backup .ZIP files and see what are the largest files. A lot of them are routine files stored by programs I haven't even used since the last backup. Others are various unimportant caches and the like.

I check the other files in the associated subdirectory; if there doesn't seem to be anything important I exclude the directory from future backups. The second backup I made (first increment), having added or changed nothing significant, was 8.9GB. After excluding just 10 directories I didn't care about (Microsoft WebCache, Firefox startupCache, all of the Google Chrome program directory, and other things relevant to my system), the backup size has varied between 35 and 300MB, and I am still fine-tuning.

This is file backup; I also occasionally make complete system images (with Acronis True Image rather than Windows Backup as it happens). The image lets me restore a system to the way it was a while ago; Microsoft Backup lets me restore anything significant I have done since the image backup. (I don't do a lot with huge video files and so on.) If I have to reinstall programs such as Firefox, Chrome, LibreOffice, it's not a problem.