See how her boobs are round, but the bikini is flat on the front of them? Her bikini isn't covering her sideboob. This top was designed for a girl with much smaller boobs, they'd be capable of actually bending around a smaller pair. These look like they're stretched over the front, because they are. The bridge (part connecting the cups) also looks to not even be in contact with her chest, because her top can't go all the way down around her boobs. She's probably wearing a sister size with a bigger band and smaller cups, which means the part of the top designed for her underbust is stretching over her boobs.

If she does very much physical activity, the edges of the cups will dig into her boobs, maybe painfully, and there's a risk of slipping if she goes swimming or diving. Bras are designed so your boobs go in the cups, not entirely behind them. This picture doesn't look comfy at all. She might be under the impression she's a C or a D, when her actual cup size is much larger. This is what happens when our society pretends D is the biggest bust size. Thanks to decades of women being misinformed about their clothing, many people have been convinced that this appearance here is just what a big bust looks like in a bikini. So some people go for this look on purpose. That's valid, but it isn't comfortable.