Cobalt also has a larger nose drawn as an upsidedown U.
Ahhh . . . I believe that - and other Cobalt/Astro differences - are the result of this:
Cobalt more closely resembles Tenma - that larger nose would suggest his adult nose would also be more pronounced (Tenma's isn't as ginormous as Ochanomizu, but it's considerably large). Also - his eyes are shaped more like Tenma's (long, with smaller pupils), and his build is long and slender (gawky) - as is Tenma.
Astro more closely resembles Hoshie - all her features are softer, and Astro's appearance reflects those things - large eyes, high cheeks, and more balanced proportions to his body.
What I wonder about is how and why Tenma reworked Astro's appearance so much - (I do have a theory - which I'll post in the head canon thread)
@Little Brown Fox - hey, if you'd like to use this as your avatar, that would be fine with me (I was only going to use it for short time, but everytime I see it, I just smile - it's so funny! :lol . Let me know!