Nanomaterials are characterized by extremely large surface-to-volume ratio. Extracellular Vesicles (EVs) – which have been recently recognized as the universal agent of intercellular communication, being involved in many physiological and pathological processes and in interkingdom biochemical communication – are nanoparticles, but this key aspect has never been rationally addressed. Here we report the first attempt to quantify the membrane-to-lumen partition of proteins in EVs. A semi-quantitative model based on available well-established compositional and microstructural data is formulated. The model allows for estimation of the overall protein content of an EV as well as of the partition between membrane (surface) associated and lumen (bulk) contained proteins as a function of the EV size and shape. It further identifies 180 nm as a key diameter, below which EVs result composed by more membrane than luminal proteins. At larger diameters the partition is reversed, reaching predominance of luminal proteins (> 80 %) in large EVs (diameter > 800 nm). The model is successfully tested to analyze and describe a real preparation composed of subpopulations of small EVs (diameter < 200 nm), including exosomes and ectosomes, and large EVs including large oncosomes (diameter > 1000 nm) from human prostate cancer cells. These findings provide basis for better colloidal description of EV samples, might help to understand the stoichiometry of proteins in distinct EV sub-populations and will improve design and interpretation of experiments, including EV engineering and dosing in-vitro and in-vivo.