memory allocation beyond 1GB datasize in linux emulation?

I want to allow the datasize of a linux binary to grow up to
1800MB.

As root I set the hard limit on datasize to 2048m, but when
I then su
to my user account and check the hard limit I find it's
only 1024m. I
tried defining a login class that has a datasize of 2048m
and use 
'su -c ...', but it didn't do it.

I ended up running the binary as root. Watching 'top' I
saw it grow to
a size of ~1047M and then it reported that memory allocation
failed.

I'd be thankful for any hints on this...

Marko