constructing and mounting a porpax

[Giannis K. Hoplite]

why do we listen of ancient writers talking of the "heavy aspis"

Perhaps because they did not pick them up all that often. I forget which author wrote degradingly that his young hoplites could not hold the aspis above their knees, but holding anything up in a bent left arm that you are not used to is taxing.

You're right. It was Aristophanes who said this. And he does not compare the aspis with another type of shield, just stating that it's hard for an untrained arm.
Khaire
Giannis