Requesting memory as a resource with torque scheduler

Hello,

Is the following request supposed to work fine with Torque scheduler?

qsub -I -lnodes=1:ppn=4 -lmem=16000mb

I have several nodes with a total memory of 6 GB, some with 12 GB, some 
with 16 GB and some with 40 GB. But the above request always puts me in 
one of the nodes with 6 GB total memory (and available is less than 
total, so no way it has 16 GB free). Can someone tell me what is going 
on here?

Thanks,
Prakash