How Does the Size of a Transaction Affect Its Total Fee, Even with a Constant Fee Rate?

With a constant fee rate (e.g. 50 sat/vB), a larger transaction size (more vB) will result in a proportionally higher total transaction fee.

The fee rate determines the cost per unit of space, so a transaction that consumes more space (is larger) must pay a higher total fee to maintain the same priority rank in the mempool.