I had a look at a couple of Croquet/Cobalt images to get a better idea of their memory requirements.
I started up a clean image and ran the Croquet Master demo with the following results:
Croquet 1.0.18 initial memory usage approx 37Mb, once the master demo was running it seemed to stabilize around 68Mb (this is the official image I think)
-- Utilities vmStatisticsReportString reports memory 38,152,380 bytes
-- SmalltalkImage current vmVersion reports 'Squeak3.8 of ''5 May 2005'' [latest update: #6665] Squeak VM 3.8.12b5'
Croquet 1.0.25 initial memory usage approx 106Mb, stabilized around 134Mb (unofficial image?)
-- same vm as above
cobalt-base-current-build-20081026 - initial memory usage approx 45Mb, consumed all available memory and crashed the VM within 30-40 minutes. Despite this, Utility vmStatisticsReportString reported a consistent memory usage of 56,776,124 bytes throughout, possibly indicating the memory leak is inside a primitive or external library. I suspect the .BVH animation may be partly responsible for this issue, but I haven't dug into that yet.
-- Smalltalk vmVersion reports 'Squeak3.8 of ''5 May 2005'' [latest update: #6665] Squeak VM 3.8.12b5'
The above results were obtained on an i386 MacBook with Intel integrated graphics running OS X 10.5.5.
The OS X Activity Monitor reports that the VM for the croquet images was running 3 threads and the VM for the cobalt images was running 5 threads.
I'm also surprised that the 1.0.25 version of croquet is using almost twice as much memory as the 1.0.18 version. The size of the 1.0.25 image file is 93.9Mb and the 1.0.18 file is 25.7Mb.