It's actually about a mix of multiple aspects in the design. From the dimensions of the audio circuit, over the need for matching battery capacity, to demands of the users (How many flagship compact smartphones do you see on the market?).
Our only truly smaller Android DAP is M3 Ultra, which has to have a significantly simplified audio circuit do to its size and smaller battery.
For M6 Ultra, M8, M8T, M9, they are all pretty much the same size, with just a small difference on height.
If you want the best sound in smaller package than our full-size Android DAPs, take a look at ONIX XM5, which runs our MTOuch system.
And ONIX will also be coming out with a thinner Android player, based around ONIX XM10. But once again, its thinner size will make it give up on some other aspects and it will not be a match to M8T or any of the M9.