It is the same platform. That doesn’t mean the frame is identical.
Toyota has said in a number of interviews that the GA-F platform allows them to scale up and down the thickness of the frame in different locations, allowing them to tailor the strength as needed. In addition, the wheelbase of the frame can be stretched as the 4-door, long-bed Tundra has a much longer wheelbase than the 300. Given that longer wheelbase, I suspect the Tundra has to resist a much larger bending moment (due to the higher leverage from the longer distance). So it wouldn’t surprise if the Tundra has the thickest/strongest steel sections in the frame.
You get it.
Being the same platform doesn't mean that they share the same frame, suspension, steering, drivetrain, or any other components. There's just a lot of people out there saying that it's all the same (or asking someone to prove that they aren't all the same). TNGA is about bringing deeper collaboration and consolidation to the design, manufacturing, and delivery of vehicles that existed previously in the Toyota Production System. And as part of that broader program they focused on some basic design improvements (lower center of gravity, improved visibility, strong/more rigid frames). It simplifies the whole production process from beginning to end. But, correct me if I'm wrong (
@OGBeno ), that doesn't mean that the underpinnings of all the vehicles in a segment of the platform (TNGA-F, for instance) are all the same. Each platform leaves enough flexibility for model differentiation for the vehicle use and market.
And the conclusion I'm drawing is that Toyota recognized that the US market didn't need the full robustness of the flagship for the US market to sell a boatload or two of them. Enter the 250/550.
"TNGA is a program of structural innovation, starting with changing the basic architecture of vehicles. Through TNGA, we are completely remaking the powertrain unit (engine, transmission, HEV unit) and basic frames (chassis). With integrated product development, we anticipate a rapid improvement in the three areas of basic driving performance, namely, moving, turning, and stopping. In doing so, we hope to continue providing cars that our customers cherish."