How come teen girls were skinnier back in the 2000s than in 2010s and today’s world?

Thick girls are in. Are they still in.
My sister was SKINNY when she was a teenager. She was a teen in the 2000s until 2007 when she turned 20. She gained some weight after high school, then lost it and then when she had kids she gained it back. She still beautiful. She isn’t skinny like she used to be but she said she even liked the thicker side of herself.
No girl wants to be skinny and guys don’t like skinny ladies. Am I right?