Ladies, I have not personally had this issue - however, I work in a photo lab and we are having many (many) Brides come in with the same problem... when it was one, it could have been an accident but in a month we've had about 10 Brides or families of a wedding party and the weddings are varying in location and state so it isn't an isolated problem.
When you sign your photography contract, you need to have a clause about the mega-pixels and size of your photos if you are planning to buy the rights to your images.
When we have had these brides come in, they have their CD of images that they have the C.R. release to reproduce except we cannot produce the images in any size larger than about a wallet because the photos are so zipped and compressed that they can't be enlarged successfully.
Basically the photographers are manipulating these ladies to believe that by buying their rights they can actually produce the images but in reality the photographers are expecting them to come back and be forced to print through their (expensive) services.
I know that in at least one case this is exactly what the photographer did, she specifically told the bride, that yes she had the right to the "images" (as stated in the contract) but she hadn't released the right to "enlarge or alter" and had prohibited her from doing this in the saving of the file.
A good rule of thumb (but isn't cut and dry, each situation is different), if you're picking up your pictures and you have over 100 images on a disk, they may not be large enough files to produce enlargements. And a huge red flag should be if you have several hundred on a disk - no possible way will you be able to do much with that!
Disclaimer: I am not suggesting that all photographers are doing this!! I just want everyone to be aware that this happens and to go in with their eyes wide open! If you don't cover this in your contract, the photographer is well within their rights to do this, after all - they are retaining the rights to their work.
Beka Lou