Hierarchically porous silica was prepared via a sol-gel route accompanied by phase separation, using low-molecular-weight poly-(ethylene oxide) (PEO) as a phase separation inducer. The interplay between gelation and spinodal decomposition was investigated in situ through the combined use of ultra-small-angle X-ray scattering (USAXS) and X-ray photon correlation spectroscopy (XPCS), enabling simultaneous characterization of structural and dynamic evolution. The silica-rich domains are initially formed by ramified cluster aggregates, which progressively grow and reorganize into more compact structures as gelation proceeds. Arrest of the transient phase-separated state occurs when the gel network structure becomes established, dominated by superdiffusive dynamics, and constrains further growth of the separated domains. Increasing PEO content induces earlier gelation and earlier arrest of the phase-separated transient state, leading to thinner silica skeletons and smaller macropores in the final material. The results establish how the initial composition modulates the sol-gel transition and the arrest of separated domains, which control the pore size. The insights obtained in this work contribute to a deeper understanding of how gelation and phase separation govern the development of hierarchical porosity in silica materials, which is critical for designing materials with tailored structural and functional properties.