Upvote:3
The rates depend on the contracts between service and network providers. These will of course vary based on the companies involved and depend in part on the expected amount of traffic set down in the contracts.
It's like any other business deal, higher volume tends to make for lower prices per unit, especially as the overhead (administration cost etc.) per unit is lower for the contract partners.
Thus 2 large providers in countries with a lot of mutual traffic can negotiate lower roaming fees than 2 small ones from countries with a small amount of traffic.
Of course there can also be politics involved (like the EU forcing providers to charge below cost roaming rates for traffic generated inside the EU, with the result that rates for all other traffic generated by EU customers go up to compensate for the losses the networks incur from roaming customers).