How does voxel_downsampling work ?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hi,
So I am subscribing to a ros topic that gets me point cloud of shape N x 4. I use voxel_down_sample to down sample the point cloud. I use 0.2 as voxel_size. It is unclear if the final point cloud will be 0.2 x original size ? I get different shape of point cloud every time i down sample even if my point cloud are always the same shapes ...

How is the voxel size applied to the point cluod ? the documentation is not clear it says : Voxel size to downsample into.
http://www.open3d.org/docs/0.6.0/python_api/open3d.geometry.voxel_down_sample.html

thank you