Disk usage of one user keep going up EVERY SECOND !!! No file change in home directory

Guys. I encounter a very strange issue. I have created a new user account on virtualmin and the disk usage report for this user keep going up every second til the maximum quota for that user. It just keeps going up while the home directory of that user doesn’t contain any large file when I check.

Home disk space used (including sub-servers) 3.05 GB (3.48 GB by server administrator, 12 kB by mail / FTP users)
Space used by databases 234.47 MB (234.47 MB in this server, 0 bytes in sub-servers)
Total disk space used 3.28 GB

Below are info when using quota -vs and quota -gs (it goes from 3GB to 6GB in just a minute!!!)

Disk quotas for user bobil (uid 1033):
Filesystem space quota limit grace files quota limit grace
/dev/md3 6749M 8192M 8192M 33380 0 0
root@fr:/home/bobil/public_html# quota -gs bobil
Disk quotas for group bobil (gid 1011):
Filesystem space quota limit grace files quota limit grace
/dev/md3 6435M 30720M 30720M 33384 0 0

When using ncdu on home directory of this user, I only get
— /home/bobil -------------------------------------------------------------------------------------------------
370.6MiB [##########] /public_html
80.7MiB [## ] /bk
7.0MiB [ ] /tmp
156.0KiB [ ] /etc
68.0KiB [ ] /Maildir
16.0KiB [ ] /homes

When I use the feature Limit and Validation -> Check Disk Quota for this user, the total goes back to 370MB again and then it keeps going up AGAIN!

How can I find the file that has size keep going up? It’s not in the home directory of this user. I’m thinking about the apache log file or access log file but when i check these files, they are small, not big.

Do you have any idea why? Thanks in advanced

I think I tracked the cause. When I use htop to display all processes, I see a CRON for that user that has been there for 21h. I killed this process and look like the disk space is not going up again