A sheet pan warps because the middle heats and expands faster than the rim, not because the metal is defective. It sits flush against the oven rack and takes on heat immediately, while the rim lags a few degrees behind — and a thin sheet has nowhere to put that mismatch except a bow. Most of it settles back flat once the whole pan catches up to oven temperature.
Why the middle bows before the edges do
Aluminum expands more than most kitchen metals when it heats up — roughly twice as much as steel for the same rise in temperature. On a flat sheet, that expansion has to go somewhere, and the middle of the pan is the part in direct contact with the oven rack, so it heats first and grows fastest. The rim sits a little higher and a little cooler, held back by its own shape, so it expands less over the same span of seconds. The sheet can't stretch two different amounts and stay flat, so it does the only thing it can: it buckles, usually right in the center.
Gauge decides how bad that gets. A thin, light pan has less metal to spread the heat through, so the gap between its hottest and coolest points opens wider before the whole sheet catches up to one temperature. A heavier pan carries more mass, and that mass acts as a buffer — heat has farther to travel and more material to average out across, so the temperature gap across the sheet never opens as wide. Once the pan reaches a uniform temperature, the stress mostly relaxes and a bowed pan pops back flat on its own. Pans that warp on every single use eventually take a permanent set, because the metal gets pushed past its elastic limit a little more each cycle.
Why a dark pan burns cookie bottoms
An oven heats food three ways at once: hot air moving around it, radiant heat coming off the walls and elements, and direct conduction wherever food touches the pan. A pan's finish decides how much of that radiant heat it soaks up. A dark, matte surface absorbs infrared efficiently and runs hotter than the surrounding air, sometimes by a wide margin. A light, polished pan reflects a good share of that same radiation instead, so it tracks closer to the oven's actual air temperature.
Food sitting on the pan feels that difference directly, through conduction, at the one surface actually touching metal. A dark pan pushes the bottom of a cookie through browning and past it while the top — heated mostly by the surrounding air — is still pale. That's the whole reason the same recipe, same oven, same rack position produces a scorched bottom on a dark pan and an even one on a light aluminum sheet. Nothing about the batter changed. Only the pan did.
Why crowded food steams instead of browning
Browning needs a food's surface to climb well past the boiling point of water, and water in the way stops that from happening. Vegetables and meat shed moisture as they heat, and on a pan with room to spare, that vapor rises straight into the oven's air currents and gets carried off. Packed edge to edge, it has nowhere to go. It collects in the gaps between pieces instead, and the air right at the food's surface turns humid.
Water has to finish evaporating before a surface can climb past 212°F (100°C) into the range where browning actually happens, and a locally humid pocket slows that evaporation down. The surface stalls near the boiling point instead of climbing past it, so the food sits there sweating in its own runoff rather than crisping. Spread the same pieces out with real gaps between them, or split one crowded pan into two, and the vapor has somewhere to escape — which is the entire difference between roasted and merely heated.