There is no harm in using a larger diameter bore, only in using a smaller bore. Larger bore pipework looks more ugly as it shows up, and it costs more as there is more expensive Cu in there - perhaps he had some 22mm that fell off the back of a lorry. All one has to do is balance the rads (supplied by 22mm pipe) by restricting the supply more by turning in the lockshield valve.
But more worrying is the prospect of what you can't see a bodger has down with his various attempts at installing stuff.