I was surprised also during the preview that the 2020 Roadster will have a 200 kWh, mostly because of the additional weight,
and also considering the constant improvement of the cells. So there must be some specific reasons to justify it.
Or is it for justifying the price, otherwise a 120 kWh Roadster could have been sold around the price of a 100 Model S or X.
Note: I assume that the next generation of Model S and X will get a 120 kWh. So I think that a Roadster with a single 120 kWh
would be also more optimum to keep cool than a dual 100 kWh batteries packs placed on top of each other.
Regarding cooling, the cells are coupled to coolant tubes on their sides, so having the heat generation spread across more cells would increase the cooling ability of the pack (more cell -> more tubes, more cells-> less heat per cell).
Pack size is likely due to the desired discharge rate for longevity, 250 MPH is a lot of power, even if you are aerodynamic (made up numbers) At a drag coefficient of 0.23 and a frontal area of 2m^2 that is 770kW output. So with a 120kWh pack, it would be running at a greater than 6C discharge rate.
Better write up:
Tesla’s Next-Gen Roadster: A (speculative) technical look at the car that will “smack down” gasoline powered cars