One way to definitely bend the 15" case, is grabbing it one-handed, lid open, by the ethernet port, with your thumb on the speaker grille, or right over any of the ports. The aluminum over there is quite thin. Or, grabbing it by the DVD tray. Just don't pick it up like that over the speaker grilles.
The 13" I don't see much of an issue since it's lighter.
The 13" I don't see much of an issue since it's lighter.