I thought as you do too but then I went out and looked at the drawings and noticed the following:
Width (from outside of muffler to outside of carb): SPE26 = 144mm, 20EI = 183
The 20 does have a velocity stack on it, but I doubt it is ~= 40mm long
Height (from bottom of engine to top of spark plug - sorta): SPE26 = 125mm, 20EI = 160
The drawing for the SPE doesn't measure to the exact tip of the plug, but it appears to be only 5 - 10 mm below
Length (from back of engine to rear of prop): SPE26 = 110mm, 20EI = 120mm
For the SPE another 10mm or so has to be accounted for due to the threaded shaft
The SPE info was taken from :
http://speengines.com/spe26.php
And the Zenoah 20EI was taken from the user manual on horizon.com
I must be missing something here since logic would suggest that the 20 would be smaller, but it looks like the spe26 is. The muffler doesn't appear to be that much thicker than the spe (certainly not enough to account for the difference mentioned above) but maybe the drawing isn't to scale.
Thanks,
Russ