Browning or searing is affected by pan temperature. Overcrowding the pan when scaling up a recipe lowers the temperature, leading to steaming instead of browning.
When you brown or sear ingredients, you're aiming for the Maillard reaction, a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor. This reaction requires high heat and a relatively dry surface. When you scale up a recipe, especially with ingredients like meat, you increase the moisture content in the pan. If you overcrowd the pan, the heat from the burner is used to evaporate the excess moisture instead of browning the food. The temperature of the oil or pan drops significantly, and the meat steams instead of searing, resulting in a grey, less flavorful product. Browning in batches ensures that each piece of meat has enough contact with the hot surface of the pan to properly brown without lowering the pan temperature too much. This allows for optimal flavor development and texture.
Ensure your pan is properly preheated before adding any ingredients for browning. A cold pan will immediately drop in temperature when ingredients are added, hindering the browning process. Use a heavy-bottomed pan to help maintain a consistent temperature.