How come i look fat but im all muscle?

i wear sweatshirts everyday because if i wear a t-shirt then it looks like im chubby and sweatshirts make it look like i dont have man boobs. i swear to god im not though. like when people see me with a shirt off they are AMAZED. like i mean really surprised. i want the girls to know that im all muscle. any advice on what i should do? wear? etc.