Just tried with mine and it's perfect for resting a medium egg or a large egg in it lain on it's side. Definitely would stop an egg rolling off a table or around. However, a hot boiled egg in glass? The glass might crack so that may be a design fault if eggs were indeed the idea.
Not optical. My design is quite complex and gilded and appears exactly the same size back and front.
I can't see why they would be salts though? The design would never be seen as would be under the salt all the time.
Whereas with an egg you could see the design quite easily once picked up to shell it.
I'm on the side of the eggs

I think the portraits were a way of personalising the (?) egg cups.
I eat my eggs in a standing upright egg cup but a family member who is Dutch eats her boiled egg left rolling around her plate with her ham and bread roll and then shells it whole when it's cooled a bit.
Interesting!