A lot of the pics of them at their best involved getting a professional photo taken, or were clearly prepared for the camera with makeup, hair, etc. done while the picture of them 'now' is just a front cam photo from their cellphone while they're relaxing at home. They're not trying to look their best in the 'now' photos vs their 'then' photos.
But then some of them just gained weight likely because they were doing sports in high school and you literally have a class that requires you to run and exercise 5 days a week. If you weren't into sports(many women are not) and you didn't like PE(a lot of people didn't) then once you were an adult on your own you didn't exercise 5 days a week a minimum of 30 minutes a day any more and didn't play on any teams. Weight gain is pretty much a guarantee at that point especially if you weren't eating well as a teen too.
And also because they're women, chances are they got into a relationship with a dude who didn't particularly care that much about her appearance. For all the flak men get about being shallow, I think men are generally really accepting of different body sizes and shapes. I'm not the only one who's seen a regular dude walking around with a broke woman built like a pastry bag or Baron Tamago while I rarely see a man who looks like Danny deVito with any woman let alone an average one.