Light competition is a fundamental force driving tree height growth and secondary succession. However, the costs and benefits of light interception and growth at the individual tree level remain poorly understood, largely due to the technical challenges of measuring light interception in natural forests. By adopting in situ measurement of light interception with a novel analytical framework, we decomposed the individual relative growth rate (RGR) into two components: light interception efficiency (LIE), defined as light intercepted per unit of aboveground biomass, and light use efficiency (LUE), defined as biomass gain per unit of intercepted light (that is, RGR = LIE × LUE). Using this analytical framework, we analysed individual tree growth rates in relation to light interception and use to examine how light competition drives size variations and its consequences for forest development during secondary succession. This study was carried out in 12 forest stands of varying ages in Hokkaido, Japan. For all co-occurring trees with stem diameters > 1 cm, we quantified growth and light interception using 3D crown geometries and forest light profiles. In young stands, RGR was positively correlated with tree height, driving rapid vertical growth and stratification. In older stands, this relationship weakened or reversed, contributing to size-structure stabilization and the coexistence of different-size species. Across all stages, taller trees had higher LIE, indicating size-asymmetric competition. Conversely, taller trees had consistently lower LUE than shorter trees, likely due to ecophysiological constraints. In young stands, the advantage of higher LIE outweighed the lower LUE of taller trees, resulting in their higher RGR. In older stands, the relative benefit of higher LIE diminished while the LUE of smaller trees increased; this reduced the growth advantage of canopy individuals and promoted the regeneration of the understory. This mechanistic framework clarifies how light competition drives forest dynamics. The transition from rapid height stratification to structural stabilization and species coexistence is governed by the shifting balance between size-dependent light interception and use efficiencies.