Use something like htop instead and it will show you the amount of allocated memory for each process.
sudo apt-get update
sudo apt-get install htop
sudo htop
Press F6 to choose what to sort the results by.
Also, have you checked to see if something may be incessantly filling up the error logs?
EDIT:
Because you have identified the culprit is using allotting a lot of space for log-files, you might want to look into using something like logrotate to help manage log-files.
Here is an official description:
Description-en: Log rotation utility
The logrotate utility is designed to simplify the administration of
log files on a system which generates a lot of log files. Logrotate
allows for the automatic rotation compression, removal and mailing of
log files. Logrotate can be set to handle a log file daily, weekly,
monthly or when the log file gets to a certain size. Normally, logrotate
runs as a daily cron job.
Of course, you can also usually change the settings of the application itself to only report errors, to only report warnings and errors, or to fully report everything.
To install logrotate:
sudo apt-get update
sudo apt-get install logrotate
For more info:
man logrotate