  Under the $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological models,
massive galaxies are expected to be larger in denser environments through
frequent hierarchical mergers with other galaxies. Yet, observational studies
of low-redshift early-type galaxies have shown no such trend, standing as a
puzzle to solve during the past decade. We analyzed 73,116 early-type galaxies
at $0.1\leq z < 0.15$, adopting a robust nonparametric size measurement
technique and extending the analysis to many massive galaxies. We find for the
first time that local early-type galaxies heavier than $10^{11.2}M_{\odot}$
show a clear environmental dependence in mass-size relation, in such a way that
galaxies are as much as 20-40% larger in densest environments than in
underdense environments. Splitting the sample into the brightest cluster
galaxies (BCGs) and non-BCGs does not affect the result. This result agrees
with the $\Lambda$CDM cosmological simulations and suggests that mergers played
a significant role in the growth of massive galaxies in dense environments as
expected in theory.
