Because modern apps are heavily bloated. Just look at the apk size of some of the most popular apps: facebook, its messenger or other apps, microsoft’s outlook, etc… The two are not necessarily connected, the apk size can be big without consuming a lot of ram, but often it is connected showing the negligence if these app’s developers, that the only goal is to churn out features and vacuum up your data at every moment possible, and saving on your resources is not at all a priority.