I think the 29x29 icon is used in search results if your application matches the search. But if there is no βrightβ size icon, 57x57 (or another available icon) will be used instead. By providing all the icons of various sizes, you have the opportunity to make sure that the icon shown by your application in all possible scenarios is exactly how you want it to look. Compare this to the way your 57x57 icon looks at 29x29 when it is simply scaled down. But, nevertheless, there is no strict requirement to include the size 29x29.
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