Prokaryotic cells are generally smaller than eukaryotic cells because they have less organelles in them. Prokaryotic cells are characterized by the absence of a nucleus or any other membrane bound organelles such as mitochondria, peroxisomes, Golgi apparatus, vacuoles, lysosomes and endoplasmic reticulum.
Eukaryotic cells contains at least one nucleus and many membrane organelles which are absent in prokaryotic cells. Also, the prokaryotic cells as a single circular chromosome but a eukaryotic cells contains 46 chromosomes that are stored inside the nucleus. In fact, the DNA is coiled and condense in the nucleus.
In summary, the lack of compartmentalized organelles and the singular chromosomes explains the small size of the prokaryotic cell when compared to a eukaryotic cell.