In CSS, we have two ways of sizing, intrinsic and extrinsic. The latter is the most common one which means using fixed values for the width or height of an element. The first one means that the sizing of an element depends on its content size.
Many icon libraries supply SVG assets that are already well-optimised. But if you’re creating your own graphics, or they are supplied by another designer, you might want to run them through a few optimisation steps.