Longyue Chen, Tao Wang, Hanwen Sun, Ke Xu, Luwenjia Zhou, Tiancheng Yang, Maxime Tarrasse, Houjun Mo, Zhaozhou Li, Yangyao Chen, Avishai Dekel, Emanuele Daddi, Xuheng Ding, Mauro Giavalisco, David Elbaz

Galaxy size provides key insights into the physical processes driving galaxy formation and evolution. Using deep JWST/NIRCam and MIRI imaging from the PRIMER survey, we investigate the rest-frame optical size-stellar mass relation of galaxies at $0.5 < z < 6.0$. We find that star-forming galaxies (SFGs) exhibit a broken power-law size-mass relation at all redshifts, with a nearly constant pivot mass ($M_{\rm p}$) of $\sim 10^{10} M_\odot$, and a slope flattening above $M_{\rm p}$. This highlights the prevalence of a population of compact, massive SFGs, likely underrepresented in previous studies. The size distribution of quiescent galaxies (QGs) is well described by a mixture power-law model, with the pivot mass increasing with redshift, from $M_{\rm p} \sim 10^{10.0} M_\odot$ at $z =0.75$ to $M_{\rm p} \sim 10^{10.5} M_\odot$ at $z = 2.6$, suggesting the minimum halo mass required to quench a high-mass QG increases with redshifts. The bending in the size-mass relation of SFGs supports two distinct size growth modes. At $M_{\star} < M_{\rm p}$, galaxy size growth is closely coupled to halo growth, while at $M_{\star} > M_{\rm p}$, an increasing fraction of SFGs decouple from halo growth and become more compact, likely associated with rapid bulge (and black hole) growth in $M_{\rm h} \gtrsim 10^{12} M_{\odot}$ halos. These compact SFGs are promising progenitors of massive QGs, as evidenced by their similar masses, surface brightness profiles, morphologies and number densities. These results suggests that the compaction pathway, rather than major mergers of extended SFGs, dominates the formation of massive QGs at $z \gtrsim 2$.