In the same way that people have different organs in their bodies to carry out different functions, cells have structures called organelles. Organelles are tiny structures inside the cell that allow for different functions like energy production and waste removal. The reason that a cell must remain very very small and contain organelles is because as a cell increases in size -- so does the distance waste must travel to exit the cell. Not only does waste have to travel a greater distance to exit a cell, but so does food entering a cell. A 1cm cube has a surface-area-to-volume ratio of 6:1 -- but a 4cm cube has a surface area to volume ratio of 1.5:1. Because there is much more available surface area on a smaller cell. Cells are able to get rid of waste and intake food more easily.