The institute have four separate hard disks for storing for storing the files and data for all students - one harddisk pr. semester. Each year the institute gets some hundred new users whose files occupy on harddisk. So the point is: Each year over a hundred new students have to share one harddisk. Each disk have capacity to ensure between 25 and 100MB pr. student.
You are able to check the amount of disk you use either via the shell or on web. When you do check you should know that these statistics are updated every night, they are not instant. If you delete several megabytes worth of files and data you are only able to see that after the next nightly update.