How does no royal charters inhibit urban growth? Is there a way around it?

See here, here, and here. 

It’s not that you can’t get urban growth without charters; as I point out in the third link, there’s plenty of historical examples where the settlement preceded the charter (indeed, it was the most common way it happened). Maidenpool, Duskendale, all these places are functioning urban centers, even if they don’t have charters. 

However, without a charter, your settlement has major limitations when it comes to trade and commerce vis-a-vis competing settlements. And if less commerce is coming through your town, there’s less need for labor in the industrial and service sectors that serve that commerce, which is going to slow down population growth. Hence why we don’t see settlements without charters the size of a King’s Landing or a Lannisport or an Oldtown.