What you see in the serial logs is only the size of the payload from a Meshtastic data message. On top of that, there is some overhead from the protobufs encoding and the Meshtastic header. Together that forms the payload of a LoRa packet that is given to the LoRa chip together with its size here. If you want to know this size, you would have to add a print statement to the code there. The LoRa chip then adds the LoRa header and preamble.
Depending on what you consider the real application data and what overhead, you can calculate the actual “goodput”.
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