[plt-scheme] memory limits in mzscheme and mred

I've got some code which progressively allocates a reasonable amount
of memory (1GBish, on an 8GB machine under debian x86_64, plt version
352, boehm GC); runs fine on mzscheme but mred -z process running the
same code dies with an "out of memory" error early on.  Is there some
difference regarding memory limits between mzscheme and mred?

John