:lisp-heap-size allocation with build-lisp-image

The documentation indicates
(http://www.franz.com/support/documentation/8.1/doc/building-images.htm):
'The (lisp heap size) value for an image is printed by (room t) output
(as resrve). '

With an output like:
Root pages: 62
 Lisp heap:  #x71000000  pos: #x71bc0000 resrve: #x9ff00000
 C heap:     #xa0000000  pos: #xa0010000 resrve: #xa00fa000
 Pure space: #xf89e8000  end: #xf9029558

Is #x9ff00000 the size of the lisp heap size? Is this a hex value
after subtracting '#x'?
If not, how is this translated into bytes?

I created two images with 'build-lisp-image' function, with one with
:lisp-heap-size of "1000m" and the other "2000m" on a 64-bit linux
system with 8gb of memory. Still the 'rsrve' value for Lisp heap is
the same, why? Does this have to do with providing :lisp-heap-start
and a beginning memory address?

Any comments/references, etc appreciated.
Patrice