Why does a Bitcoin transaction not automatically cost more when you send more bitcoin?

Detailed explanation

Since SegWit, BIP 141 limits a block to at most 4,000,000 weight units. A transaction's virtual size is derived from its weight. This limit keeps bandwidth, storage and validation manageable for independently verifying nodes but makes on-chain block space scarce.

On-chain payments usually use a chosen fee rate, often quoted in satoshis per virtual byte, or sat/vB. The total fee equals fee rate multiplied by virtual transaction size. A transaction with many inputs can therefore cost more than a payment transferring a higher bitcoin amount with only one input.

When many transactions wait in mempools, they compete for block space. Miners or pools select transactions under their own rules, often considering fee revenue and dependencies. A higher fee rate can make earlier inclusion more likely but guarantees neither a particular block nor a fixed confirmation time.

Wallets can estimate fees for a desired confirmation target. These estimates rely on observed data and may be too high or low when demand changes. A very low rate can cause long waits or limited relay; an unnecessarily high rate raises costs. RBF and CPFP can help in certain cases, depending on the transaction and wallet. Network fees must be distinguished from a provider's trading, service and withdrawal fees.