From Forum Nokia Wiki

this article had explained about the effiecent way of loading the image and mask of image include <yourmif.mbg> in .cpp filed AknIconUtils::SetSize initializes the icon to the given size, if the parameter aBitmap is an instance of CAknBitmap, i.e.created with AknIconUtils methods. If it is not CAknBitmap, this method does nothing. Note that this call sets the sizes of both bitmap and mask (if it exists), regardless of which is given as the parameter. Note Without a call to SetSize Bitmap will not be drawn and seen on the screen

Although CreateIconL have overloaded method for creating only the single bitmap rather than its mask but this has been more efficient interms of size and loading