Behaviour of +RTS -Hx -Mx -RTS options on win32

All,

I have a program which runs to completion on windows in about 90Mb of
heap space. Ie the following works ok:

prog.exe +RTS -M90m -RTS

However if I use the -H option to reserve a bigger initial heap size:

prog.exe +RTS -H60m -M90m -RTS

then the program fails part way through with heap exhaustion. This seems
odd given what the -H and -M options are supposed to do.

What particularly makes me think this is a bug is that the following
also fails:

prog.exe +RTS -H730m -M750m -RTS

and it fails ever sooner if I use:

prog.exe +RTS -H750m -M750m -RTS

I'm using GHC 6.2.2 on Win2k3.

If this proves hard to reproduce, the program in question will be
included in the Gtk2Hs 0.9.8 1st release candidate which should be
available in the next few days.

Duncan