When something or someone is thinner in a dream than in real life then it usually means that something is beginning to carry less weight with you in real life. Ask yourself what is diminishing in your waking life. Is it a certain relationship? Certain feelings you once had. Is it an issue that is not as difficult as it used to be? Are you losing your concern or care for someone or something?
If YOU are thinner in a dream than you are in real life it can mean that you are lightening up. Perhaps life doesn’t feel as heavy as it used to. If you are actively trying to lose weight, then your dream is giving you encouragement by placing you in a thinner body so that you will keep working on your diet and your health.