Something high in the planet's atmosphere is absorbing optical radiation, causing the planet to appear larger when observed in optical wavelengths. This has been interpreted as evidence for the presence of titanium oxide (TiO) and vanadium oxide (VO) in the upper atmosphere. TiO and VO are strong absorbers of radiation, especially in optical wavelengths. The presence of TiO and VO should have a significant effect on the atmosphere of WASP-121b as they are expected to generate a strong thermal inversion. Basically a thermal inversion is a layer of hot air overlying a layer of cooler air. The lower density of the hot air prevents the cooler air below from rising.
Evans et al. (2016), "Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere", arXiv:1604.02310 [astro-ph.EP]