I may be wrong since I wasn't a part of the team when we did the stating, but the reason for the hoplite shields having a higher value was due to it's construction rather than just its size. On size alone, the scutum and thureos would have higher values, but the aspis is bronze-faced making it far sturdier and heavier. For example, we know from Carrhae that the recurve, composite bow can have arrows penetrate the scutum's leather coverings and layers of wood (although the layering does not seem to have always present). I can think of no similar circumstance with the aspis despite its long history fighting against similar weapons.