  Characterizing the structural properties of galaxies in high-redshift
protoclusters is key to our understanding of the environmental effects on
galaxy evolution in the early stages of galaxy and structure formation. In this
study, we assess the structural properties of 85 and 87 Halpha emission-line
candidates (HAEs) in the densest regions of two massive protoclusters, BOSS1244
and BOSS1542, respectively, using HST H-band imaging data. Our results show a
true pair fraction of 22+-5 (33+-6) percent in BOSS1244 (BOSS1542), which
yields a merger rate of 0.41+-0.09 (0.52+-0.04) per Gyr for massive HAEs with
log (M_*/M_sun) > 10.3. This rate is 1.8 (2.8) times higher than that of the
general fields at the same epoch. Our sample of HAEs exhibits half-light radii
and Sersic indices that cover a broader range than field star-forming galaxies.
Additionally, about 15 percent of the HAEs are as compact as the most massive
(log(M_*/M_sun) > 11) spheroid-dominated population. These results suggest that
the high galaxy density and cold dynamical state (i.e., velocity dispersion of
<400 km/s) are key factors that drive galaxy mergers and promote structural
evolution in the two protoclusters. Our findings also indicate that both the
local environment (on group scales) and the global environment play essential
roles in shaping galaxy morphologies in protoclusters. This is evident in the
systematic differences observed in the structural properties of galaxies
between BOSS1244 and BOSS1542.
