EOS m has

5184x3456 pixels on 22,2mmx14,7mm

with 1280x720 crop mode you use 5184/1280 ~ 24,69% of the horizontal and 720/3456 ~ 20,83% of vertical pixels.

14,7mm*0,2083 ~ 3,06mm

22,2mm*0,2469 ~ 5,48mm

sqrt((3,06mm)²+(5,48mm)²) ~ 6,27mm

if my calculations are correct you'll use a diagonal of ~ 6,27mm with 720p

43.2mm/6,27mm ~ 6,89 <- this should be the effective crop factor

compared to bmpcc which has a super 16mm diagonal of 14,54mm and 2.97 crop factor, it's a really tiny sensor. less than 1/4 of effective sensor size.