Thanks for the reply. Why do you think the certification would be different? They're working with the same frame, it's just the lengths that would be different.
If I'm not mistaken, the EPA (and CARB in California) requires a separate certification for any combination of chassis, transmission, engine, exhaust, and a few other components. So a certification for a Silverado and one for a Tahoe would have to be pursued separately even though the vehicles are similar to our eyes.
If you're curious,
here is the EPA webpage that lists the certification data for various categories of vehicles.
Here specifically is the spreadsheet that lists the light-duty vehicles that are currently certified. Each row on this spreadsheet represents one "certified vehicle" as viewed by the EPA, and there is a mind-boggling number of variants of the Chevy pickup alone.
Apparently it only costs about $25,000 per variant to actually apply for the certification with the EPA; the cost is in the development and testing that the car manufacturer goes through in order to ensure the variant will pass.
The market would be smaller just as a function of the market for SUVs being much smaller than the market for pickups; I think they would sell proportionately as well when you consider overall sales for each segment.
It would only matter whether the vehicle would project to sell sufficiently to meet profitability requirements that are part of the investment analysis. A vehicle could be seen as selling very well indeed on a market-share basis, but if it's a money loser the company won't greenlight it, unless it's a halo car or something. This is exactly why there are so few wagons on sale in the US now, for instance.