> biguser|User requiring more memory:\
>         :datasize=2048m:
> 
> then I su'ed to root and did
> 
> su -c biguser marko
> 
> but the hard limit when I'm marko is set to 1024m

I just tested this on NetBSD -current using su -c and it worked correctly 
for me. The soft and hard datasize were 2147483648. (I tested with and 
without my class defined in master.passwd.)

What version of NetBSD are you using?

Can you show output of your testing?