I had a special font colour coded in for the author and call number sorts (while the rest of the data is default black, the data sorted by is a different colour on the display) and hadn't entered it for the title sort file. 'twas the font codes that were making the files different sizes!
Thank you to anotheranon, tommdroid for their useful advice and to hadesgirl (whose response wasn't helpful in the technical sense, but did make me laugh). Now that the mystery is solved, we can all sleep soundly. :P
P.S. I've added a few items to the music lists section and am now open to the usual suggestions from the users ... any notions for what you'd like to see? (yes, I HAVE thought of track listings, but I'll have to play around with the HTML generator and see if I CAN include them)