[fastball]

I think they're actually all by height, and if you google them the OP actually matches up. e.g. googling

"duck height": 20 – 30 in (51 – 76 cm)

"goat height": 16 – 23 in (40 – 58 cm)

Maybe it would be better to do them by mass.

[d13]

Goats average 75cm https://www.dimensions.com/element/goats Ducks average 12cm https://targetstudy.com/nature/animals/duck.html

[Nition]

A little unfortunate if they just used the automatic Google info results. The duck one is the Indian Runner duck - a Mallard would make more sense per the emoji.

[hanniabu]

Is your mouth height 15cm/6in?