So i came across this site and you can see these lines on the first and second paragraph:
For a crude but easy effect, draw the bright portions of the scene (eg, light sources) into an FBO, then downsample it using GL_LINEAR minification several times. For the final render, simply combine the original scene with the downsampled FBO's.
In the following example, the original scene is 128 x 128 and is downsampled 3 times. This requires 4 framebuffer objects. The original scene is shown in the upper-left and the final rendering in the upper-right.
Is there any reason i should downsample multiple times (128 -> 64 -> 32 -> 16) rather than downsample it once (128 -> 16)?
If you scroll down more, they even downsample it like 9 times. (128 -> ... -> 16 -> 128 -> ... -> 16 -> and so on). Why not do it like (128 -> 16 -> 128 -> 16 -> and so on)?