The first question has to take into account the fact that Egypt is a desert country, surrounded by sand. It is known that in pre-dynastic times the dead were buried in rectangular or oval pits dug in the sand and that the superstructure was unlikely to have consisted of anything more substantial than a heap of sand supported at the sides by a wooden frame. The superstructure of the royal tombs at Abydos, from the reign of Djer onwards, were mounds of sand supported at the sides by walls of mud-brick. The hieroglyph for the Egyptian word for a pyramid (m(e)r) shows a true pyramid supported by such a wall or frame (see hieroglyph).
My explanation is at odds with the idea that the Pyramids represent the rays of the sun or the primeval mound from which land emerged, both of which are fanciful.
The question of why the Giza group of pyramids, especially the Great Pyramid, are so huge has to be addressed with the knowledge that they were found to be empty. This is usually thought to be because they have been emptied by robbers. However, it is possible that the Great Pyramid was always empty and that even its sarcophagus was always empty. There are several arguments against the Pyramids being tombs but, according to one writer in 1989, educated travellers and antiquaries are generally in agreement on the nature of the Great Pyramid; ‘all consider it to be the tomb of Osiris’. That explains its uniqueness and its size.
Osiris was the Egyptian god of the dead. By building a cenotaph for his spirit to live in, the Egyptians ensured that their god dwelt with them.
For my full article on this, go to this link: The origin and purpose of the Pyramids.