Let's say computer S (sender) sends some IP packets to computer R (receiver). Between S and R, there are 6 networks, divided by 5 routers. So the IP packets leave R, go through network 1, pass router 1, go through network 2, pass router 2, etc, eventually going through network 6, and arriving at R.
Let's say from network 1 to 3, the MTU of the networks decreases each time. From network 4 to 6, the MTU increases each time.
The IP packets get split up in network 1 if they are larger than the MTU allows; they also could get split up again in network 2 and 3. But from now on, the MTU of the following networks increases. Does that mean that the IP packets get put together again? Or do they keep their small size until they arrive at R?