Testicles are underdeveloped. Is it a problem?

Yes. Even the subjective feeling that your testes are too small for your age or not working properly, or that your body is not developing normally with your peers, is something you should talk about with your physician at your next annual.
Hypogonadism/infert. Simple blood test of your testoterone and a semen analysis will let you know if there really is a problem...Things can still be normal even with "smaller" testes...But he tests will really tell the answer.