When sun light falls on my bathe tub i noticed that the shadow of any small particle floating on the water surface is bigger than the particle and also it is quite circular i.e. Deform from It's actual shape. Generally it tends to become a circle.
In this case, the likely explanation is that the surface of the water near the "floater" is not flat due to surface tension. This causes the light rays entering the water near the particle to be bent, causing a "shadow" under the particle.
You might have also noticed that the swirling water when the drain is opened casts a "shadow" (without the presence of any particles) because the water surface is not flat.
For example, you can see the bending of the water surface in this photo.
And here is an example of the cast "shadows".