Also, we already know the Ghast is "4 blocks large". We have no idea what that means exactly, but hear me out.
If we assume that 4 blocks large means this Ghast head is 4x4x4, then this is is how it looks like within the game (I think this is the max size of slimes?):
But PLEASE, do NOT take this as authoritative in any way, we have no evidence for any of this. It's just a wild guess.
I know Notch said it wasn't ghost-like, but maybe he had a different idea of "ghost-like"
Hear me out.
If the thing in the corner is the body (one block), and the head is 2 blocks, and the Ghast floats one block, that would mean he would technically be 4 blocks high.
This thing is crying right?
And if it's like 02 from Kirby like that one guy said, it would have wings on it, like an angel.
The ghast may very well be the weeping angel like monster Notch wanted to put in. That's why he has such a hard time killing it.