This isn't necessarily true.
If a 10m pop area has a 70-30 split, it's still better to campaign there than a 5m pop area with a 50-50 split. In absolute numbers it's still 3 million supporters versus 2.5 million supporters.
If a 10m pop area has a 70-30 split, it's still better to campaign there than a 5m pop area with a 50-50 split. In absolute numbers it's still 3 million supporters versus 2.5 million supporters.
Under your model where the goal of the campaign is to visit your supporters, you'll favor smaller cities that strongly support you over larger cities that support your opponent. A place with 5 million population that's 70-30 in your favor has 3.5 million supporters.