Why Is the Fee Calculated Based on Transaction Size (Bytes) Rather than Value?

The transaction fee is calculated based on size (bytes or vbytes) because the cost to the network is related to the consumption of the scarce resource: block space. Block space is measured in data size, not monetary value.

A transaction transferring a small amount of cryptocurrency takes up the same amount of block space as a transaction transferring a large amount, provided their data structures are identical. Charging by size ensures that the fee accurately reflects the resource usage and maintains the economic incentive for miners to prioritize based on efficiency.