Documentation indicates : "If the maskSource and the source have different dimensions, the maskSource image is stretched to match the source size."

It is a problem if we want to mask an Item in another larger Item. For example, if we want a ProgressBar with a rounded Rectangle background (see image with light orange progress bar), it is impossible to set a mask with the background size and which has the light orange Rectangle as a source, because the mask will change its width according to the light Rectangle width which matches the ProgressBar's value (as we can see on the images with the white progress bar)