Xen users - Apr 2013 - expected dom0 memory usage

I set GRUB_CMDLINE_XEN="dom0_mem=1024M" and put "(dom0-min-mem
1024)"
and "(enable-dom0-ballooning no)" in xend-config.sxp.

Then in upgrading the machine''s RAM, MemTotal in dom0''s
/proc/meminfo is
906864 with 4GB, but only 779900 with 16GB.  Does that seem reasonable?  
Is Xen hiding part of that 1024M from dom0 to...manage the larger 
memory?

This is 4.1.4.