Transaction weight/size understanding

Question

According to Monero source code there are two metrics for the transaction size:

• Size https://github.com/monero-project/monero/blob/v0.17.2.0/src/wallet/wallet2.cpp#L870
• Weight https://github.com/monero-project/monero/blob/v0.17.2.0/src/wallet/wallet2.cpp#L878

There is Wallet RPC method returning them both estimate_tx_size_and_weight: https://github.com/monero-project/monero/blob/v0.17.2.0/src/wallet/wallet2.cpp#L14187 and https://github.com/monero-project/monero/blob/v0.17.2.0/src/wallet/wallet_rpc_server.cpp#L4317

I'm trying to understand how that metrics are related to real transaction size, for example:

• https://localmonero.co/blocks/tx/82966ee2ade89b159b80bc376b608cc9e8e8cd3b5206abcac267984414fee8f0 real size: 1452 estimated size: 1460 weight: 1460 - both differs by 8 bytes
• https://localmonero.co/blocks/tx/46a36725ed36e624fc8de94ed79943284639d4e524d600b0367297f7c575bc01 real size: 1966 estimated size: 1969 weight: 1969 - both differ by 3 bytes
• https://localmonero.co/blocks/tx/de4b645e96be1d98b2e753a450b433b5ab2916c2911c0a4a6736cbe43a4624bf real size: 3700 but estimated size: 3253 weight: 7221 - it seems no size and no weight are related to real size because so big difference

What's wrong with the estimation or maybe I don't understand some logic behind?

Answer 1

In your 1st and 2nd examples, where the difference is tiny, you're talking about a few bytes difference between estimated and actual size. I'd expect to see this due to things like varints. Also, weight and size estimates for these match because the bulletproof clawback isn't being calculated/applied due to there only being 2 outputs on those txs.

In your 3rd example, you're seeing a big difference between estimated weight and real size because this tx has 16 outputs, hence you're seeing the bulletproof clawback come into play on the weight estimate. Also, you're seeing the several hundred bytes difference between real and estimate size because that tx has a tx_extra field which is 547 bytes in size and the estimation only uses the common case of tx_extra being 44 bytes in size.