trigger warning : discussion of marginal gains to follow

i have two helmets - a giro syntax and a specialized s-works evade. the evade is newer and lighter, but is much larger when viewed from the top. it fits much looser as well, requiring more tightening of the clicker. when i measure my head size, i'm right in the overlap between the M and L but the M felt too tight, like i could barely get it far enough down. riding, though, the largeness of the evade is noticeably annoying, and i keep moving it up so that my sightline when i'm head down-ish is clear.

the evade is supposedly a much more aero helmet... but i'm wondering if it's actually better or worse for the helmet to be larger than necessary. obviously the drag of the head/helmet is less with smaller frontal area and the same cD, but wouldn't the general principle of streamlining suggest that a huge helmet with a smoothly curved frontal surface and long trailing edges actually be better, since otherwise that same air would be hitting the much-less-aero shoulders, chest, and back of the rider?

any interesting recommendations on lightweight and aero helmets that offer more or different size increments than the evade, which is S-M-L? they list M as 55-59cm, and L as 58-62. my badly done head measurement was around 58, but as noted the M didn't seem to fit. the syntax M is also 55-59, with the L 59-32, but it does seem to have a more round shape.