The vagina of women is adaptive, flexible, and muscular. A lot of men and some women, incorrectly assume that the vagina is like a rigid inflexible structure with very specific limits. Nothing could be further from the truth.
1) A woman might seem shallow, because her previous boyfriend was small or she has no or very little sexual experience. The vagina adapted, relatively, to the small size of his penis.
If she had a boyfriend with a thicker or longer penis, almost all of the time, her vagina will adapt to this size too. The human female vagina was made to accommodate the ranges of sizes of nearly all human male penises. The exceptions are extreme cases, like a very tiny midget woman and a man with an extremely unusually large penis. These extremes, males with a penis bigger than 9 inches (23 cm) and women shorter than 4 feet 8 inches (146 cm), are very unlikely combinations.
It's just that it can take a little bit of time, depending on the woman. Some women can adapt instantly, while other women could take 1 week (sex encounter 3 to 5 times), and the size difference between the previous and new guy is a factor. I know this as a swinger, so have seen and experienced this. Say a woman's ( she is 5 feet 2 inches in height/158 cm) previous boyfriend (or present male swinger partner) has a penis of only 4.5 inches (11.5 cm), the new guy is 7.5 inches (19 cm). To the new guy, when he first has sex with the woman, her vagina might feel shallow. He might at first touch her cervix easily. That's because her vagina is used to the short penis. But after having sex with the new guy a couple of times, the vagina will/can adapt to the new size. Literally, the vaginal canal will/can stretch and get longer, to stop the discomfort of the penis head slamming into the cervix so strongly. So where the longer penis guy was easily touching her cervix at first, it can be a bit harder to do so later or feels like his penis can barely reach the cervix.
And the vagina doesn't get "loose" (because it's muscular), especially if a woman knows how to control her vaginal muscles. She can switch back to the smaller penis by gripping it. With experienced swinger women, they can handle different size penises with almost no problem. Usually the difference between the guys isn't that much. X guy is 5 inches (12.5 cm) and Y guy is 6.5 inches (16.5), so just 1.5 inches (4 cm) difference. In the bigger scheme of things, so what? Just like your hand can hold a big or small rod.
Furthermore, if the woman is tall, accommodating a larger penis is usually easier and the switch over is faster. A woman that is 5 feet 2 (158cm) might need to have sex 3x with the bigger guy to get used to, while a woman that is 5 feet 9 (175cm) might need ZERO time and can handle it right away (if sexually experienced).
2) Some women can appear to be shallow or tight, because they are squeezing their vaginal muscles excessively or holding their legs tight and pushing their hips forward in weird positions that blocks penetration.
I often find this to be about mental issues with the woman more than a physical one. That is, she is trying too hard to impress the guy that she is an almost-virgin. For her smaller penis sized boyfriend, she got used to squeezing her vaginal muscles very hard to make it tight for him, and doesn't realize that a guy with a bigger penis does NOT need OR does NOT want her to do that.
Among the most ridiculous sexual experiences that I ever had, a few tall women stand out. I was fingering this tall woman (5 feet 11 inches/near 180 cm) with wide hips. She is even taller than me. I could easily put 4 fingers in her vagina. And probably could fist her, with some gel. Anyway, she was not realizing how many fingers that I was putting inside of her. When I would attempt to put the penis in, she would go through this big act like my dick was going to kill her. My penis is bigger than average, but not 4 fingers wide, so this woman was clearly getting carried away with her acting. She was also doing retarded things with her hips and legs to make pumping and penetration extremely difficult, and if I was not so horny and determined, I would have got up and walked out. The 2nd time we had sex, the middle of me having 4 fingers in her vagina and side ways (a bit grouped together, which she was loving it as I stroked her G-spot too), I told her how many fingers I had in her. Chick FREAKED OUT. Even she tried to stop me putting that many fingers in her and kept asking how many fingers do I have in her. Long story short, it took me having a couple of sex talks with this woman to get her mind right and to calm down worrying so much about how big her vagina was. By the way, her vagina felt very good and had a nice grip. She just had to stop being overly worried and relax her legs.
Not to mention that vaginas can stretch out so that little babies can get out of their bodies...
