Massive galaxies at z ≈ 2 are 4 to 6 times more compact in effective radius than local counterparts of the same stellar mass; disks also shrink systematically toward higher z (Trujillo 2007; van der Wel 2014). ΛCDM models grow galaxy sizes through minor mergers, inside-out star formation, and feedback, but require finely tuned merger histories and feedback efficiencies to reproduce the strong size evolution of compact high-z systems while accommodating the more modest evolution of disks.
The standard model assumes galaxies grow primarily through hierarchical merging plus inside-out gas accretion, with sizes increasing as the mass increases. The observed strong compactness at z = 2 to 3 plus the modest disk-size evolution simultaneously demands different size-growth mechanisms operating at different mass scales, neither of which is parsimonious in minimal ΛCDM.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, cascade-stage proto-galaxies emerge with effective radii set by the parent cascade-stream geometry (P22, P25, P33, P34). Head-on cascade collisions (low impact parameter b) produce compact remnants with thin geometry; grazing cascade collisions (high b) produce extended disks; the cascade impact-parameter distribution P(b) ∝ b distributes the size population at deposition.
Compact red nuggets at z = 2 to 3 are head-on cascade events: low-b collisions deposited their baryonic content in compact configurations whose effective radius reflects the smaller pocket's self-gravity scale (P33: W ∝ M_min^(1/3)). Subsequent cosmic evolution adjusts sizes modestly through minor accretion and stellar evolution, but the dominant size at z = 0 reflects the cascade-deposited starting size plus this small evolutionary adjustment. Massive low-z ellipticals are descendants of cascade-stream events that produced more extended deposits.
Angular-momentum inheritance (P31, P32) sets the J/J_circ ratio that determines disk extent: high-J inheritance gives extended disks at deposition, low-J gives compact spheroidal cores. The same M1 framework that resolves the JWST early-galaxy mass crisis (recid 7, 108), the SMBH-seed problem (recid 109), and the merger-rate decline (recid 110) accounts for the size-evolution puzzle. Compactness at high z is a cascade-geometry signature; modest evolution to z = 0 is the small post-cascade size adjustment.
If precision JWST + Euclid + Roman size-evolution surveys find galaxy size evolution fully consistent with hierarchical-assembly + minor-merger-driven puffing-up at the 5% level (no cascade-stream-geometry signature in size distributions), the M1 cascade-deposition explanation is refuted. The signature SCT prediction is compact red nuggets at z = 2 to 3 corresponding to head-on cascade events with sizes scaling as W ∝ M_min^(1/3).