Do jeans that reveal your backside (crack) mean they are too tight?

How do I prevent this? Seems all the designer jeans I try on reveal crack when I sit down. I'm not sure if this means I need to size up or what? Anyone have a clue what causes this and why it's so prelevant with almost evey jean I try on? I'm 5'3 and weigh 123. Curvy but not heavy?