The only justification for not doing this is protectionism. Starmer is placing party above country. We can see how damaging the Tories are. I do not want to see their likes again.
London currently has 73 MPs out of 650 (11.2%) when they have 9mil out of 67million people (13.2%).
So guess you’re right that they’d get stronger representation.
However on the other hand, people like the Green party got 2.7% of the vote in 2019 while only getting 0.15% of the seats.
Some voices get stronger, but it’s not just cities.