What Is Disc Quota / Disk Space And How Much Do I Need?
In web hosting lingo, “Disc Quota or disk space or storage size” refers to the amount of hard disk space the hosting company offers for your web pages. All your documents, graphics, animations, database, etc. all must fit into this space storage space.

Storage of Data:
Also REMEMBER that if you have a way on the site that keeps track of the form submissions from the website, or other software in the site that stores users detaile, ie names and addresses, website members or recipiants of newsletters  etc, usually in a database, then this extra and growing data has to fit into the allocated disc quota as the database size grows.
Also the website statistics grow with time and this will effect the allocation

Storage Space / Disc Quota: how do you know how much you need?
Getting an estimate for your needed storage space is as easy as looking at your local disk. See how much space your web page requires on your local disc. If you use Windows Explorer, it is easy to find out your storage space. Just click on the folder where your sites are located and choose “Properties.” The easiest way to do it is to right-click in the Windows Explorer on the folder where your sites is located, and choose Properties. Your web page will take similar amount of storage on the server as it takes on your hard disk.

Using a Content Management System [ CMS ] the following has to be considered, size of the software that drives the site, size of the template files, offer membership and other services on the site, the images and embeded videos or multimedia.

Keep in mind that you may want to add some files to your web page in the future. Therefore, you should allow for some extra storage when signing up for the web hosting service, so that new content and files will fit on the server as well.

A good rule of thumb for a static site is to buy at least 50% more storage than your website initially needs, for a CMS that has no membership and is only storing a small amount of data that is removed every month, a minimum would be 90% over the size of the cms  and database

It is also important not to confuse storage size with Data Transfer.

How to control this?
Remove old data after saving it, Remove old  files,  keep only files on the webserver that are required, make images as small as possable - remember that your viewers may not have fast internet access, this also reduces the Data Transfer.