As another commenter mentioned, this is more like a physical limit for efficiency of light capture. However they also follow a scaling law of angular resolution versus diffraction. Smaller lenses means more blur from diffraction but smaller lenses also allow better angular resolution. Many insects with compound eyes follow this scaling law, having ther same (provably optimal) ratio of eye radius to lens size, despite having very different sizes of eyes. William Bialek is a biophysicist who uses this frequently as an example.