If we have Giant Frogs doing 5 damage, I'd say it's safe to give an Emperor Penguin 4 damage. Especially considering that for a frog to be giant it could still be half the size of an emperor penguin.
Yes, but this is a fantasy game and we have no way of knowing the actual size of a "Giant Frog". For all we know it could be as big as a house. Also we don't know how it does damage. Maybe it has some kind of an attack what does not require a big size.
This is different with Penguins because Penguins are not fantasy creatures. We know exactly how small they are, how little they do damage, and how non-aggressive they are. Not only do these Penguins here have a relatively high starting attack, their ability makes them even more aggressive and strong. You made them offensive creatures, when in real life Penguins are anything but that.
Thematically, the best way to use Penguins would be to make them support creatures with a low attack. Making them offensive creatures like this just doesn't make sense to me.