So, speed of reentry is directly a consequence of surface area (or energy expended by fuel to counter, in the case of the booster which does not use friction with the atmosphere to slow down) of one side. You'll produce the same amount of heat (and sound, light, but let's keep the model simple so we can understand better) no matter how fast you come in and no matter how wide your surface area is (assuming the same mass), it's just the thermal properties of the material and the surrounding environment dictate how quickly that heat dissipates, and the surface area determines how distributed the heat is, and the speed it's entering determines how quickly the heat is generated.
So to slow down more evenly and have less heat at the max point per square inch, you need wider surface area (or you need to expend fuel firing engines in the opposite direction of travel, what both parts do at the end to slow to 0, and a problem due to the rocket equation, fuel has mass and so increases the amount of kinetic energy you must dissipate), and that means more mass and more engineering and a bigger vehicle. The goal ultimately is of course optimizing all these variables.
So to slow down more evenly and have less heat at the max point per square inch, you need wider surface area (or you need to expend fuel firing engines in the opposite direction of travel, what both parts do at the end to slow to 0, and a problem due to the rocket equation, fuel has mass and so increases the amount of kinetic energy you must dissipate), and that means more mass and more engineering and a bigger vehicle. The goal ultimately is of course optimizing all these variables.