As someone who designs toilet paper dispensers for a living (www.vandalstop.com), I can attest that there is no standard size anymore. You have your many flavors of standard rolls (of which Costco happens to be the largest, I found this out the hard way when one of my designs couldn't accommodate a Costco Roll). You have your jumbos, your mini jumbos, your cored, your coreless and everything in between.
I won't profess to know the entire reason why there are no standard sizes but I think it boils down to two things:
1. Cost savings - TP is narrower now than it was years ago by probably close to 3/4". Each manufacturer has decided to literally cut corners in a different amount.
2. Proprietary Rolls - Many of your large TP companies are in the business of selling large volume paper contracts to customers. As part of those contracts, they will give dispensers to the customer for free or very low cost. Those dispensers are designed to only accommodate their paper product so it is more difficult for a customer to switch providers without tearing out the old dispensers.
Finally, for anyone interested in a bit of history about toilet paper, this site is an interesting read:
http://encyclopedia.toiletpaperworld.com/toilet-paper-history/complete-historical-timeline