As long as the CPD is selected to suit the smallest CSA then there is no reason why CSA cannot be reduced as a circuit progresses.
Could not have said it better. There is no reason why any circuit could not be in different sizes along its length and long as the smallest csa can carry the load, eg 25mm to a 16mm to a 1.5mm to a 50w gu10. This is what happens from a sub station to your house. The question is about protection of the circuit. Select the correct rated fuse for the smallest csa and your ok. Closest I can find in the brb is 133.2.2 page 19 but even that doesn't give us a defined answer. Good point about putting too much stress on terminals. Sounds like this is domestic, in your case I would change it all to 2.5 (ring) or all to 6mm radial to stop any confusion for the next sparky.