Not knowing anything about the origin of the files the only thing I can
suggest is that once a Comment has been manually resized it retains those
dimensions regardless of how much content is later added or removed. I
haven't witnessed any "automatic" constraint imposed on Comments when
opening .xls files from PCs or Macs in Excel 2008, so I don't know of any
other possible cause than the manual resizing possibility.
You should be able to restore the elasticity of any Comment if you display
it in Edit mode then go to Format> Comment - Alignment & replace the check
for Resize to Fit Text. If there a many that have to be changed you may be
able to use an Apple Script to automate the process but I can't offer
anything there - my scripting skills are quite lacking
In general, make sure that you have Office fully updated (12.1.5) & that
you've repaired disk permissions just to be on the safe side.
HTH |:>)
Bob Jones
[MVP] Office:Mac