This is not an act of worship. The entrance to a tomb is not a major thoroughfare. The larger the aperture, the greater the effort to hew it and the larger the stone needed to close it. Needing to be closed by a stone rolled into place, the lower edge of the aperture needs to be about the elevation of the ground. So you have a low opening of small diameter through which a man can crawl and a body may be dragged. The inside of the tomb can be as spacious as can be afforded.