1. Genetics.
2. High enough estrogen levels makes them darker, higher prolactin makes them pale.
3. Tanner stage four development
4. Simply more volume behind them WILL make them wider.

I have observed all four in action and have been pleasantly surprised. They took a jump in width after my first fat transfer and second one is very likely to do the same. Over all volume development has helped a lot and when Tanner 4 kicked in, that's when the biggest change happened so far. That's pretty much it, be careful with tattoo, that has to be absolutely spot on and remember that your skin pigments can change with age and hormone balance, so it might not always look the same.