Ultrasmall nanoparticles (NPs, <10 nm) have promise in cancer treatment, yet little is known about how NP physical properties influence penetration through solid tumors. To elucidate the role of NP size and structure, we prepared a series of sub-10 nm poly(amidoamine) (PAMAM) dendrimers and gold NPs (AuNP), and evaluated penetration in multicellular tumor spheroids (MCTS). Smaller generation 2 dendrimers (G2-NH, 2.9 nm diameter) penetrated 2.5-fold deeper than larger G7-NH (8.1 nm) (P = 0.0005). Despite increased accumulation within MCTS, electrostatic cell interactions and ligand (folic acid, FA)-mediated targeting had minimal influence on penetration. NP rigidity played a minor role in penetration, with smaller rigid AuNP (2 nm) penetrating significantly more than larger AuNP (4 nm) (3-fold, P = 0.014; G2-NH vs. G4-NH, 2.8-fold, P = 0.033). Our findings highlight the importance of rational NP design and provide design cues for tailored NP distributions within solid tumors.