Assuming that you actually meant video content, I think your question may be a bit misguided on the nuances and goals of video encoding. Video encoding can be both lossy and lossless. Lossless video encoding isn't particularly interesting in most cases, but I do believe that HEVC (H.265) will usually come out slightly smaller. However anything to do with encoding will always vary based on the actual source content. So partial answer to your question would probably be x265, but it depends. Based on the source you could construct theoretical content that could be better tuned to one or the other's encoding strengths.
Where it gets interesting is in lossy encoding. With lossy encoding you seek to retain visual acuity to a certain standard while minimizing size and/or processing requirements. Both codecs do an excellent job at removing the right amount of information to effectively fool the human observer. With lossy encoding there isn't really a filesize difference, as you tune your filesize to whatever you want to given your source and your desired output constraints. The big feature of av1 is that it is open and unencumbered by patents+royalties and will hopefully therefore make it THE industry standard in the coming years. It's openness also makes it more likely to eventually be ubiquitous as it should be implementable and playable on most new video platforms and hardware, and hopefully the mythical one format that just works everywhere.
