bigger volume = more money for R.Mail to transport the package
Even if it is for the same weight of package as you get less packages per van load with larger items so you have to pay more for all that extra space you’ve used up
Which incidentally is why you get silly sized packages from CRC, they’re maximising the amount of packages they can get into a van by making them all a uniform size, therefore reducing their packaging costs and maximising their profits
Or something like that, anyway it’s late and I can’t believe that someone doesn’t actually understand this basic principle of logistics