The right outfit, the right hairstyle, and a square jaw can do wonders (also unique to his Elezen face is the less gaunt cheeks - both of these would be nice options, absolutely, but they don't actually change the proportions).
Just go into the character creator and toggle between the race and job attire with Gladiator selected on a male Elezen and you'll see an example of what I mean.
That's always been a thing with Elezen anyway - any outfit and hairstyle that hides the longer neck and head proportion to the rest of the body makes the difference significantly less noticeable. It's why certain hairstyles are really, really popular on Elezen.
Pics to show the outfit/hair/etc impact.
Change the attire, nothing else.
Throw in the fuller hair of Aymeric and his square jaw, and that's why it looks like he has different proportions.