what's Mimble Wimble got to do with blockchain size?
Not quite sure, but I think in MimbleWimble, full nodes eliminate most of transaction data overtime as they only store unspent outputs indefinitely. But I haven't read the potential disadvantages of this. By the way, it has already been implemented in Litecoin.
How about people get tired of a huge blockchain that's terabytes in size?
What does a hard fork have to do with that?