Top up hydraulic brake fluid?
Your brake is a closed system, and your brake cable is filled with a mixture of brake fluid and air. Air is compressible, while brake fluid isn't. So, the larger the proportion of air in the system, the more compressible it will be, the more effort is required to actually stop the bike, and this is frequently described as "spongier".

Now, in your specific case, you need to judge whether you have enough stopping power for the environment/conditions in which you ride. If the answer is "No", then topping up the reservoir (i.e. replacing air with brake fluid) might make sufficient difference.