Urban green space is increasingly treated as a core planning asset because it supports heat mitigation, stormwater regulation, biodiversity, and public health. Yet cities also face structural pressures that constrain land available for vegetation as populations and built footprints grow. This article provides a planning-oriented synthesis of evidence on how green space quantity and spatial structure change with city growth, drawing on a globally distributed sample of 150 cities and a published remote-sensing workflow that constructs concentric-circle scalograms around standardized city centers. Green space is derived from Landsat-based NDVI using a city-specific upper-quartile threshold and evaluated with three landscape metrics: green share, mean patch size, and green connectedness. Across cities, reported green share increases with population but follows sublinear scaling (exponent 0.26), indicating declining per-unit green availability as cities become larger. Mean patch size and connectedness also increase with growth, but with patterns that vary by income and climate groupings. At the within-city scale, most cities exhibit statistically significant power-law relationships between scalogram extent and green space metrics, providing cross-scale validation of recurring spatial growth patterns. The findings translate into planning-relevant benchmark targets: if green infrastructure is expected to scale as a universal urban service, strategies must counteract the baseline tendency for green space to grow more slowly than city size.