if dont use copy2ram, then the opt.sqfs is not copied over the network and the size doesnt matter. it also needs less resources, you have a lot of free ram if you do not copy the opt.sqfs.
if you still want a smaller opt.sqfs, then use fluxbox as window manager. fluxbox does not need gnome and is fast and good. you can remove all gnome and gcc related programs and libs. i think without gnome and gcc you will get a opt.sqfs with maximum 100MB maybe less. to get a really small and clean opt.sqfs its better to compile xorg, fluxbox and rdesktop again. i dont know, maybe i create it.
regards
elmar