Is it a problem to be 25 and still have small teeth?

No. Unless the small teeth are primary teeth, small adult teeth that are healthy and in occlusion should not be a problem. Retained baby teeth can be a problem when they are lost.
Not primary teeth. Are you an outlier? By this i mean, are you the only one in your family with small teeth? Some people have bigger teeth than others. As long as you have your permanent teeth and they are in good health, you have nothing to be concerned about. What has your dentist said about the size and condition of your teeth?