Out-of-focus bright spots often appear as large bright discs because the light from a small highlight is spread into a larger blur circle. You notice these discs because they add visible light to the image. Dark or non-bright areas are also blurred, but spreading “no extra brightness” over a larger area doesn’t create a visible glow, so they attract less attention.

There’s also a recording limit: film and digital sensors have a maximum brightness they can reproduce. On film, heavily exposed areas can reach full density; on digital sensors, pixels can saturate at full well capacity. Once that maximum is reached, extra light won’t record as a brighter value, so bright out-of-focus areas can look like flat, bright blobs rather than showing more detail.

So the effect is mainly a combination of defocus spreading highlights into blur circles (bokeh) and the medium’s limited ability to record brightness.