Cell size — when internal environment becomes unstable

its surface area to volume ratio becomes too great

Explanation

A cell becomes too large to maintain a stable internal environment when its surface area to volume ratio becomes too great — or more precisely, when the surface area becomes too small relative to the volume. As a cell grows, its volume increases faster than its surface area, reducing the ratio of surface to volume. This limits the cell's ability to exchange nutrients and waste fast enough to sustain its interior, which is why most cells remain small.