There's valid technical reasons for having a coaxial octo within a total craft body+arm size radius of 1.5 meters as well. You can generate more thrust and carry more payload for the same disc loading area.

Four large motors and four 28" CF props produce less thrust than four arms, each arm with two motors stacked, and eight props.

Octos are not a conservative, redundant, fail tolerant design decision just for the sake of protecting expensive gear. Many of them still have 1+0 non redundant power systems.