The vaginal barrel is essentially a muscled structure throughout. During arousal, a combined psychological and physiological process occurs, known as "vaginal tenting." The mucosal membranes, Bartholin's glands and other anatomy produce lubrication, intended to aid penetration, and improve semen viability. The vaginal barrel expands to accommodate a penis... during tenting, the vaginal barrel dialiates and enlarges, and in some women, the cervical neck "tips", increasing the gaps around the fornix. Many women find direct contact with the cervix uncomfortable, or painful, usually during tenting endorphin response reduces this discomfort. For well-endowed males, penetration into the anterior and/or posterior fornix becomes possible. Many women experience intense orgasms from this form of penetration.
Vaginal tenting is not permanent - the vagina does not "stretch" or increase in size/diameter and remain so - over several hours, after sex, stimulation, and arousal cease, the vaginal barrel returns to a relaxed state, absent the lubrication and dilation that are part of the tenting process.
Lot's of what gets described in many of these posts is imaginary - by men - who have little idea about female anatomy, physiology, and sexual response. Indeed, many women enjoy the sensation of being penetrated by a big cock, especially a girthy cock. It can be a different physical sensation, especially if her familiar partner is average or below average endowment. That said, there's no permanent change to the pussy after sex with a big cock.
There's lots to this, especially for partners who have very different body types (very petite woman, physically large, endowed male). There is sometimes temporary damage, like tearing, fissure injury, however unless the injuries are severe (trauma from very rough/vigorous penetration) it will heal within a week to ten days.