A frame designed around the driver's feet
How a hard human constraint drives the geometry of everything else.
A frame is designed around the driver's feet because pedal position, footwell width and the angle a human ankle can comfortably hold are constraints that cannot be negotiated the way a tube diameter or a bracket thickness can, so the chassis designer fixes the footwell first and then routes every structural tube, every triangulated panel and every suspension pickup point around that space.
Packing a boot around the pushchair
Packing the boot of a car around one large, immovable item, a pushchair or a spare wheel that will not fold any smaller, shows the same ordering in a familiar setting. Every other bag gets shaped and squeezed to fit whatever space is left once the pushchair has claimed its share, and nobody packs the small, soft bags first and then tries to force the pushchair in afterwards, because it is the one thing in the boot that cannot be compressed or folded into a gap it does not already fit. A driver's feet occupy the same role inside a chassis. Tubes, brackets and pedal box mounts are the flexible cargo, and the feet are the one item that gets its space settled first.
A foot cannot be traded for stiffness
Most of a chassis design has room to trade one thing against another. A tube can be made a little thicker for more stiffness at the cost of a little more weight, a bracket can be moved a few millimetres for easier access at the cost of a slightly longer load path, and a panel can be left untriangulated for now and reinforced later if testing shows it needs it. A human foot offers none of that flexibility. It has a fixed length, a fixed width, a limited range of ankle motion, and it needs space to move quickly between pedals without catching on anything.
Drivers also differ, so the constraint arrives as a range. Anthropometric data describes how much feet vary in length, width and ankle flexibility, and a footwell sized too tightly will pinch a larger driver's feet against the pedal box under braking, while one sized too generously wastes width that could have gone toward a narrower nose. The usual answer is to size the footwell for the largest foot the rules or the driver roster require, then build every surrounding member outward from that boundary, since narrowing or widening a footwell after the surrounding tubes are welded is far harder than drawing it correctly in the first layout sketch.
How the decision spreads through the car
A team that lays out its main tubes first and fits the footwell into whatever space is left usually ends up with a driver whose ankle twists to reach a pedal, or whose shoe catches on a lower rail under hard braking, none of which shows up on a drawing until a real person sits in the finished car. Routing a structural tube slightly off its ideal straight line to preserve the footwell costs a little stiffness, in exchange for a car the driver can drive at the limit without fighting the pedals.
The footwell's influence also reaches well beyond the pedal box. The angle of the steering column, the height of the seat (set so the driver's knees clear the wheel), and even the position of the lowest tube running past the driver's hips all route around the volume the feet claimed first. A late discovery that a larger driver needs a few more millimetres at the toes can therefore ripple backward into the seat mount, the column angle and sometimes a structural rail that was never meant to move once welded, which is why the fixed human constraint is settled before any of the flexible structural decisions are made around it.