Cell size is tightly controlled in all cells, suggesting that having a specific size is critical for cell function. Interestingly, there is a strong correlation between cell size and DNA content between different species, but also between cells of the same species. These observations suggest that maintaining a constant DNA:cytoplasm ratio is critical for cell function. How this relationship is established and why it is important is poorly understood. Using reversible cell cycle blocks in yeast and human cells, we were able to show that a large increase in cell size in the absence of DNA synthesis leads to an impaired DNA damage response that severely impairs genome repliation and drives cells into senescence. In addition, we find that the DNA of enlarged cells becomes more prone to breaking, suggesting that chromatin organization is fundamentally altered in enlarged cells. Together our data indicate that a low DNA:cytoplasm ratio is detrimental for genome maintenance and cell fitness.