It's not the hide, it's the sheer mass.
See, a knife might well puncture rhino hide. But a knife wound is a pinprick to something that big.
And yes, bigger things can take more damage than smaller ones sometimes. Like you say, bigger wound. But it also takes more to put down a bigger thing. Which is why I'm not suggesting elephants be more vulnerable to anti-tank weapons than I am. I'm just suggesting that they not be much less so.
The basic argument doesn't need that point though. To summarize what I'm really trying to say:
1. In order to model how hard it is to punch an elephant to death, elephants need Toughness.
2. If elephants have Toughness, then the large weapons that ought to kill elephants easily become unable to kill elephants easily.
3. Therefore, large weapons ignore the Toughness of elephants.