As someone who's worked at a photostore with a mini lab...
Bigger images take more time to scan, hence are less profitible unless they charge more for them. If you were really serious about image quality, you would invest $15,000 in a drum scanner.
And lets face it, the majority of people getting picture CDs done are using their $200 uber-zoom point and shoot that has a lens just adequate for a 4x6 print.
Photo CD's, standard scans from Frontier minilabs (100k-200k) aren't going to be great. They're going to be good enough for printing 4x6's at 300dpi, or good enough for viewing at 72 dpi on your monitor. Why? Because this entire market is geared towards efficiency and consumers, not professionals or Prosumers for that matter.
To my understanding Frontier Mini Labs (which are really, really popular now) are designed to scan primarily as JPEGs or BMPs (I think, I might be wrong) if that's what they used.
How many grandparents with 15 gazillion pictures of their new grandkid is going to know what to do with a 30mb TIFF anyhow?
Plus 24 images at 200k a piece isn't even 5 mb on a CD, but they're not really charging you for the CD, they're charging you for the scanning time and interruption in workflow because they probably have several dozen more one hour jobs to crank out behind you.
/rant
Whatever. I just brought up a whole host of issues, but I hope this enlightens.
I don't mean to sound evil, but if I come across that way, yes, I am evil.