Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background-free luminescence. The progress in synthesis nowadays enables access to complex core-shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core-only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near-infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF4(20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF4 shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb3+ and 2% Er3+. The impact of size, composition, and core-shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.