Honestly, it's just the way the shirts seem to fit her normally from the chest up and then are also fitted to her tiny waist that feels a little off to me. Like finding shirts for big boobs is famously a struggle. Either it's big enough for the chest and then it's tent city underneath or you get something snug and the fabric around the chest stretches. I guess that amount of realism would be a bit much for a manhwa, and looking at the last chapter I appreciate how her shirt is bunched up under her chest.
I came into this just wanting to push back a little against the idea that being thin with boobs that large is so unrealistic that it's bad anatomy. But I also couldn't deny that there's something off-putting about her side view, so I tried to be more specific about what bothered me.

Why are there people out here complaining about their body proportions get out of your house, touch some grass and observe your surroundings because people with that body proportions actually exists