A critical question in evaluating urban agglomeration policies is whether regional integration translates core-city digital dominance into broad-based growth and regional convergence. Using the successive enlargement s of the Yangtze River Delta (YRD) as a quasi-natural experiment, this paper employs a balanced panel of 284 Chinese cities (2003-2022) to estimate these effects. Through comprehensive difference-indifferences , event study, mechanism analysis, and spatial models, we demonstrate that YRD enlargement significantly elevates the urban digital-economy index (UDEI) and reduces the relative digital-economy gap (UDEG), a conclusion supported by extensive robustness checks. The dividends of enlargement are particularly salient in cities characterized by stronger entrepreneurial vitality, more polycentric structures, higher digital-economy attention, and earlier accession cohorts. Furthermore, causal mediation analysis reveals that the of knowledge breadth reorganization and new-technology space reorganization drives this digital catch-up. Moderation analyses reveal that local absorptive capacity-specifically, superior talent-capital matching and technological proximity-amplifies the digital dividends of YRD enlargement. Spatial analysis reveals that YRD enlargement generates a short-range local-shadow effect on UDEI, whereas its effect on neighboring UDEG is only directional. By contrast, digital institutional proximity produces significant convergence-oriented spillovers in UDEG. Ultimately, this study advances the literature by shifting the focus from average growth outcomes to the nuanced distributional, competitive, and spatial dynamics of urban digital transformation