In case you were wondering why the LibreOffice icons look too big on the app grid or dash, this is why: They're the full height of the canvas, instead of using the smaller portrait base shape.
The reason for those base shapes is that we need to balance circular, rectangular and other shapes optically in size (e.g. a circle with the same width as a square looks smaller).
It also allows for more creative shapes, e.g. having an overlaid object (e.g. a pencil) stick out of the rectangle.