For reasons best known to the recesses of my brain, I'm thinking of this in terms of chairs.
As I see it, a nice but strong person is likely to have a sturdy structure underneath the upholstery. It's comfortable and supportive, and it can withstand some abuse - though smacking it might get you a bruised hand when you discover the padding's not as thick as you expected.
It may have give, and it may be flexible rather than totally rigid, but it'll withstand a real attempt to break or warp it into something it's not built for. In contrast, the internal structure of a pushover will collapse under pressure like a cheap deckchair. Sure, it's light and portable, and comfortable for a while, but I wouldn't advise leaning too heavily on it.
Of course, it can be hard to tell the difference until you put those internal structures/principles under stress. Moral of the story:
Sit on people.
...or not.