Mushy chick? Newborn Duckling with very wet poo and lethargic.

[WolfGirlMagic]

Here is a size comparison of her to her sister

 

[WolfGirlMagic]

As for weights on my ducklings
I actually have surprising variability in the weights.
Dodo (the sick one) is only 60g
Bearnaise is 90g
Nibbles is 102g
L.Quackington is 103g
Tipsy is 143g
Puffin is 160g
Zippy is 182g
And Goldie is a whopping 193g
I will get weights tomorrow to see if Dodo is growing, and at what rate compared nto her siblings.

 

[AquaDuck]

She seems most eager to eat during her monitored socialization with the others.
O think she cues off of them

Ducklings are social creatures and act more like one unity than individuals, so no surprise she only eats with the others. As long as she's eating enough for her digestive system to function properly, I wouldn't worry about the amount. When she's properly healed and healthy enough to be put with the others permanently, her appetite should increase.
Great that she's feeling better!