> In fact, it is never even a flat fee despite the fact that it does not actually cost anything more to transfer a larger number over a network than it does a smaller one.

A flat fee doesn't inherently make more sense either, as the marginal cost of processing a transaction is negligible. The costs come from the infrastructure.

There are many ways they could price it, but relating it to transaction size makes a fair amount of sense, since people making larger transactions are demonstrably able to pay more.