My understanding is that this is not necessarily true. The Dolby HD format does compress the audio stream more - resulting in less disc space occupied by the lossless track, HOWEVER, it's not backwards compatible with older equipment, so a second, lossy Dolby Digital track must be included alongside the Dolby HD track, thus negating some of the data storage benefit. DTS-HD MA, on the other hand, is a "core + extension" format which includes the lossy DTS track as the core (for backwards compatibility) and a residual stream which describes the differences between the original track and the lossy version. Consequently, Blu-Rays with the DTS-HD MA audio track don't require any additional digital surround tracks in order to comply with the Blu-Ray spec (which specifies that at least one Dolby Digital (AC-3), DTS, or PCM track be included). In the end, it's not as clear cut which audio format is "more efficient" in terms of disc space - there are simply too many variables at play.