Why are Kerbals eyes so big?

[pandaman]

It makes them look cute, and cute looking things are more likely to trigger the 'parental' instincts in other species which means they are less likely to get beaten up or eaten.

[Mjarf]

They still get beaten and eaten by the Kraken...

[T-Bouw]

Their eyes are big so they can collect more light and see (more) celestial objects (better).

It's only natural since they are a spacefaring species.

[FirstSecondThird]

Kerbals, I believe, live underground, without a lot of ambient light, and because of that, they need bigger eyes to see, kinda like how our pupils dilate whenever there's a low amount of ambient lighting, so that way we can at least see the silhouettes of objects instead of their details, allowing us a certain extent of navigation in the darkness.

[Skorpychan]

It's what happens when 'little green men' meets 'bug-eyed monster'.

[Tw1]

Because they're full of secrets. 

 

Their eyes are also their nostrils. Little known facts/conjecture