The maintenance of cell size homeostasis has been studied for years in different cellular systems. With the focus on ‘what regulates cell size’, the question ‘why cell size needs to be maintained’ has been largely overlooked. Recent evidence indicates that animal cells exhibit nonlinear cell size dependent growth rates and mitochondrial metabolism, which are maximal in intermediate sized cells within each cell population. Increases in intracellular distances and changes in the relative cell surface area impose biophysical limitations on cells, which (...) can explain why growth and metabolic rates are maximal in a specific cell size range. Consistently, aberrant increases in cell size, for example through polyploidy, are typically disadvantageous to cellular metabolism, fitness and functionality. Accordingly, cellular hypertrophy can potentially predispose to or worsen metabolic diseases. We propose that cell size control may have emerged as a guardian of cellular fitness and metabolic activity. Cell size is intimately connected to metabolism and growth rate, which are influenced by mitochondrial activity and biophysical constraints. We discuss that cellular fitness is often cell-size dependent. While the current evidence relies largely on proliferating cells, this connection from cell size to fitness may also help explain metabolic diseases. (shrink)