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The process to create a small icon from a big one, is to not just reduce its resolution by a standard downsample, but to recreate the icon from scratch with gradually fewer details. As your downsampling shows, there are too many details in the higher resolution icon, that do not translate well to a smaller image, as there are not enough pixels for each of them. Thus, you need a simpler model for it. Besides, the sampler combines too many pixels together with a low range of contrast, making for a very blurry image, when the purpose of a lower resolution is to retain edges and distinctive traits of the model.

For this purpose, you can not rasterize a model into a high resolution icon and then downsample that icon. You need to rasterize the model for each icon resolution level. 
When you reach a resolution where the details of the model no longer appropriately translate into the bigger grids, you will need to simplify the model. That be by replacing curves with lines, or removing features, opting for the simplest silhouette available. For the smallest resolutions, a reasonable approach is often to just hand-fill the pixels.