However, several people here are conflating "best compression as determined for a competition" and "best compression for use in the real world". There is an important relationship between them, absolutely, but in the real world we do not download custom decoders for every bit of compressed content. Just because there is a competition that quite correctly measures the entire size of the decompressor and encoded content does not mean that is now the only valid metric to measure decompression performance. The competitions use that metric for good and valid reasons, but those good and valid reasons are only vaguely correlated to the issues faced in the normal world.
(Among the reasons why competitions must include the size of the decoder is that without that the answer is trivial; I define all your test inputs as a simple enumeration of them and my decoder hard-codes the output as the test values. This is trivially the optimal algorithm, making competition useless. If you could have a real-world encoder that worked this well, and had the storage to implement it, it would be optimal, but you can't possibly store all possible messages. For a humorous demonstration of this encoding method, see the classic joke: https://onemansblog.com/2010/05/18/prison-joke/ )
However, several people here are conflating "best compression as determined for a competition" and "best compression for use in the real world". There is an important relationship between them, absolutely, but in the real world we do not download custom decoders for every bit of compressed content. Just because there is a competition that quite correctly measures the entire size of the decompressor and encoded content does not mean that is now the only valid metric to measure decompression performance. The competitions use that metric for good and valid reasons, but those good and valid reasons are only vaguely correlated to the issues faced in the normal world.
(Among the reasons why competitions must include the size of the decoder is that without that the answer is trivial; I define all your test inputs as a simple enumeration of them and my decoder hard-codes the output as the test values. This is trivially the optimal algorithm, making competition useless. If you could have a real-world encoder that worked this well, and had the storage to implement it, it would be optimal, but you can't possibly store all possible messages. For a humorous demonstration of this encoding method, see the classic joke: https://onemansblog.com/2010/05/18/prison-joke/ )