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Seems unsurprising, as your data is already compressed.

I would have expected some savings from games but mayhaps not from lz4. Music and movies though, if lz4 had significant savings to provide these would have long been added to the media formats (likely in a more specialised and effective or efficient form).

For that kind of data I am surprised LZ4 isn't making it bigger unless ZFS declines to store the compressed version when it does.

That is correct, if the first few records (a virtual block size sorta) (not sure how many off the top of my head) don't compress well, it assumes that the file is not compressible and just writes the data out skipping the compression step.

If compression was enabled after the data was written it will not be compressed until the data is written again. Just some info not sure if it applies to your compression level situation or not.