baby question -- is this normal??

[minimonkey]

Okay, so I'm watching my little neighbor hatchlings grow and develop outside my window (they are now 9 days old), peeping and flopping around, etc. So wonderful!

But -- I've noticed an increasing difference in their size. At first I just assumed that one is male and the other female, hence the difference. Now, however, one is more than twice the size of the other -- he also seems to be developing more quickly on the whole -- his eyes are always open and bright, he is growing lots of feathers, etc. The little one still has her eyes closed about half the time, and isn't fledging as fast. The big one actually moves to the outside of the nest to keep the little one safe, it seems.

Mama and Papa Pij are leaving them unattended for longer and longer periods of time, and coming back with lovely bugs for them to eat from time to time. Should I be worried about the little one? Or am I just being a nervous "first time parent?"