Thank you for your question and there is one thing that you should be confirmed of that there is nothing like a small cavity vagina. To elaborate on it, one should understand that the vaginal muscles have natural folding of skin which when needed can expand to whatever size of penis is being inserted in it. You must know the fact that a baby is delivered through a woman’s vagina only and if it could expand for the delivery of a baby then it can certainly expand for a penis no matter how big it is. For having proper sex a woman should be aroused properly because it is an important factor in expanding the muscles of vagina. It is quite clear from your query that you are being a little susceptible to the penis penetration in your vagina and you fear about the pain too much. Yes, it is a little painful for the first time but you have to overlook the pain and that can possibly happen if your boyfriend is can finger your vagina a little because that will calm you down. If it does not work then you should not do sex until you get a very desperate feeling to do it.The porn stars have a big cavity because they are those who are sexually active and their vagina has expanded permanently. Therefore there is no point in comparing with the porn stars and the sizes of their vaginas. During the first intercourse some women also bleed because of the thin layer in vagina which is known as hymen but this could be absent in those woman who do some kind of stretching work like cycling or riding It is very important to understand the peak time of having sex and feelings associated with it. Trying to have a sexual intercourse without proper intent then it could be painful. My best wishes to you, hope you will be satisfied with the answer.