I'm no fan of the CBP but this seems like a flimsy objection. There's no reason to believe why their staffing levels will grow linearly with population. They're tasked with protecting the border, so we'd expect their staffing levels to grow linearly with the amount of border that needs protecting. If there's more stuff coming in (eg. aliexpress shipments from china), or more border crossings (legal or otherwise), we'd expect their numbers to grow faster relative to population.