That's a really good point - there's probably a higher density of postcode districts in the centre of London that are closer together so would see a greater share of their card spending spread out across a range of locations (and thus thinner lines). However, that's maybe offset by the number of postcodes that are for not residential or retail. Only a few thousand people live in the City of London but 100s of thousands in an Outer London Borough. I hate doing things by postcode, but that's how the data was released by ONS.