I think you'd have to know the inner workings of excel to know why it's
growing.  And even if you had that knowledge, I'm not sure if you could stop it
from happening.  (But I've never seen the .xlb file grow that big or that fast!)

I'd suggest that you delete the *.xlb file, then recreate it so that you like
it.

Then close excel and back up that new pristine *.xlb file.  When you notice that
the file is growing, just copy the backup to the correct location (overwriting
the one that's out of control).

In fact, you may want to set up a small bat file to do this.  Then put a
shortcut on the desktop.  Click it every so often (with excel closed).

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Do you add/delete things from your toolbar--either manually or via code?  That
could be why it's growing.