Stratigraphic grain-size trends record tectonic and climatic signals. Here, we show how measurements of sediment calibre and clast lithology can be used to identify changes in accommodation space and sediment budget, using examples from Palaeogene syntectonic clastic deposits in the southern Pyrenees. We identify a mid Eocene interval of rapid grain-size fining, driven by local tectonic subsidence; a late Eocene interval of diminished local accommodation generation; and an Oligocene interval showing order-of-magnitude lower grain-size fining rates, driven by increased sediment supply. Our results demonstrate that grain-size trends provide a powerful means to explore the tectonic and climatic boundary conditions governing sediment routing systems.