What factor prevents cells from becoming the size of giant blobs that consume city blocks?

Cells are limited in size primarily due to the surface area-to-volume ratio. As a cell grows, its volume increases faster than its surface area, making it difficult to efficiently exchange nutrients and waste with the environment. Additionally, the structural integrity provided by the cytoskeleton and the need for effective communication within the cell also impose limits on cell size, preventing them from becoming excessively large.