Forwading question...disk usage ???

[Wintermute]

I have a Physical hosting plan witht he .com domain on it.
then I created Standard Forwarding domain with its .net
and pointed DNS to the .com IP

now I get an e-mail every morning from my VPS server at about 4 AM...

"Limit has been exceeded on domain 'mydomain.net' owned by client 'name' (Contact name 'Some Name'):
Disk space usage: 4.00 K
Disk space limit: 0
Traffic: 0
Traffic limit: 0 "

Now the .com is fine..i created it with 10Gbytes of space and its using only about 20Mbytes...
when I first create the forwarded .net it shows up with 0k..
then after a day or so the 4k appears...what is that ? and how do
I keep from getting the msg ?Did I set up forwarding wrong ? its fairly straightforward...

thanks in advance..

winter