Yes, it's definitely possible for a woman to be "loose". Vaginas vary in size just like penises do, and every other body part does. A small number of women simply have noticeably larger vaginas, and will be "loose" compared to the average, and a small number of women will be extra small. It happens; that's just how those people were built.
Beyond that, vaginas can be MADE looser by trauma. Childbirth can definitely do it, especially having multiple children or having a difficult birth, but also being regularly fisted or using really large sex toys/objects can stretch and tear a woman and reduce the elasticity, making her looser.
Finally, if a girl hasn't had sex for a while, her vaginal muscles will be used to being contracted all the time, and she'll seem to be "tight" the first couple of times, but as she starts having sex regularly, and the muscles get used to relaxing and stretching a bit, they get more "limber" (just like a person who stretches or does Yoga regularly), making her a bit "looser", though that really just means it's less difficult for him (and less painful for her) to penetrate her initially. Once inside, she won't seem any different. But if she goes without having sex for a couple of weeks, those muscles lose their ability to relax, and she'll seem "tighter" again (i.e., it will be harder for him to penetrate her at first) until she starts having sex again on a regular basis.
This difference is minor, and any difference will be gone after just a few days of having sex regularly. If a girl hasn't had sex in a while, and starts having sex every day, there'll be a bit of a difference between Day one and Day 3, but no difference between Day 3 and Day 180. She doesn't get progressively looser the longer she goes.