Tiny insects typically have tiny antennae, but larger in comparison to the size of the head than large insects. Tiny antennae have smaller surface area and can accommodate fewer olfactory receptors. As an adaptation to the smaller antennae, the receptors may be reduced in size. However, limited reduction is possible, generally less than 50%. As a consequence, the number of receptors is reduced. The smallest number* of recorded olfactory receptors is 38, on the tiny wasp, Megaphragma mymaripenne compared to 1000 or more in its larger relatives.
*Alexey Polilov. 2015. Small Is Beautiful: Features of the Smallest Insects and Limits to Miniaturization. Annu. Rev. Entomol. 2015. 60:103–21