Q: What exactly does the comparison mean when Jesus talks about new wine and unshrunken cloth in Matthew 9:15-17
A: Fantastic question! The context is key. John’s disciples have asked Jesus why it’s not his disciples’ normal practice to fast (which was an important part of Jesus discipleship at the time of Christ).
Jesus answers that it is because those disciples are in the presence of the bridegroom! The appropriate response to Jesus’ arrival to establish the kingdom of God was joy, not sorrow (a feast, not a fast!).
It would be as inappropriate for the disciples to fast in the presence of Jesus as it would be to sew a new, unshrunk piece of cloth to an old garment (which would result in further tearing the garment); or by putting new wine in old wineskins (which would ruin the wineskins).
Jesus is – in essence – illustrating/explaining something along the lines of: “There is a time of sorrow in store for my disciples when fasting will have real meaning (in the context of the new, Messianic age) – but now, in my presence, it is appropriate to rejoice!”