Which is the biggest single celled organism?

This video lesson from TED Ed shows why a single celled organism can’t grow bigger in size compared to multi cellular organisms. When a single cell tries to increase its size, its volume increases faster than its surface area. Lesser surface area with increased volume leads to clogging of resources to inside and waste materials to outside. Eventually the cell dies. So the ratio of Surface area to volume is very important for a single celled organism. This is the basic idea behind the evolution of multi cellularity. If a single cell could grown large in size, multi cellular organisms would never have evolved.