Matlab by default uses your Documents folder as a working directory, this is so you get the same experience across versions and computers.
This is useful if you use different machines, but can be problematic if you do heavy matlab work as it can take a sizeable chunk of your home directory quota. Of course if this directory is set somewhere local to the machine it will not follow you to other machines, neither will it be backed up, this makes it even more important that you save your work on a regular basis.
If you wish you change this behaviour, please follow the below help article from matlab, if you change this to $LOCALHOME you will still get all the security benefits of having it in your home directory, without it taxing your quota.
Should you choose to do this please be aware that there are situations where the data on /local can be destroyed or lost, do not save anything to this folder that you need backing up (see README in /local for further information)