This girl was probably smart enough to buy a tank top with one of those built in support bras, but apparently she forgot what her mother told her back in the day.
“Jenny, you are going to grow up and you will have huge boobs. The average bra will not be enough to contain your magnificent rack. You must seek out and use only the best and most supportive bras, or your breasts will swing around and bonk you in the nose.”
In other words, her boobs are too big for that measly built-in bra, and it looks like there is quite a bit of activity underneath that tank top as she hustles down the sidewalk.