Small size

I bought a t shirt Saturday so got home had it fit and it was very tight decided to take it back I sent my sister they told her that they only exchange I specifically asked if it don’t fit they said I can return it for a full refund I don’t understand why they won’t refund because I don’t want anything at the store! Nothing appealed to me but the t shirt what is the policy for refunds and exchange because of the slip is vague itself.