Newcastle and Sunderland share a catchment of around 2 million people. That includes Tyne and Wear, Co. Durham, and Northumberland.
Tyne & Wear - 1.1m
Co Durham - 530K
Northumberland - 322K
Wearside has about the same population as Teesside, but Sunderland draw a lot of support from County Durham (which alone has a population of 530K). They traditionally draw more support from there than the skunks do.
Outside of Teesside, North Yorkshire has a population of 250K and its never been a Boro hotbed like Co. Durham is for the mackems. You only have to go as far as Northallerton and you're in Leeds territory.
There was a detailed post on the old FMTTM forum years ago when this debate came up. Turns out by percentage, Boro needed 21K attendances to match the skunks and mackem average home gates of that time. Mind, that was going back a few years.