I think I just stumbled onto the root cause of the image growth. I had downloaded the new production squeaksource.6.image (copy of the Oct-5 image) to my laptop to get a "data.obj" file exported. This export process runs in the background and takes a long time, so when I came back to check on it, I found two debuggers with the message "Port not available."
What's happening is when the image is saved, it gets most of the way through SmalltalkImage>>snapshot:andQuit:embedded: so that the file is saved on disk, however, at the end it calls #processStartupList: whereupon SSRepository class>>#startUp: tells SSKom to start up. SSKom produces a debugger if you tell it to start when its already started.
So the debuggers from the prior image save are slowly piling up within each subsequent image save, a slowly growing until it reached the limit.
I guess the bug should still be there; I guess I'm puzzled why this didn't happen on box2..
I think we should work on moving squeaksource to the new image ASAP.