Nanoflake thin films made of layered semiconducting transition metal dichalcogenides such as MoSe2 are promising large-area electrodes for photoelectrochemical solar energy conversion applications. However, their energy conversion efficiencies are typically much lower than bulk electrodes. It is unclear to what extent this efficiency gap stems from differences among nanoflakes (e.g., area, thickness, and surface structural features). Moreover, a major unanswered question is whether exfoliated nanoflakes can achieve similar energy conversion efficiencies to bulk crystals. Here we use a single-nanoflake photoelectrochemical approach to show that 7% of MoSe2 nanoflakes in a thin film exhibit larger photocurrents than the parent bulk crystal used for exfoliation. This "champion" population, which is hidden in ensemble average measurements, shows that exfoliated nanoflakes can achieve similar photocurrent collection efficiencies to bulk crystals. However, there is also a large, "spectator" population, which is also hidden in ensemble-average measurements, that is mostly responsible for the overall lower photocurrent efficiency compared to the bulk crystal. Photocurrent mapping showed that photocurrent collection efficiency is lower at perimeter edges than at interior step edges. Our results highlight research opportunities to improve 2D semiconductor thin film photoelectrodes.