A graphics processing system sorts graphics primitives for rendering into lists corresponding to different sub-regions of a render output to be generated, each list indicating primitives to be processed for the render output. A primitive list building unit divides a render target into various sub-regions, determines which sub-regions a primitive falls within and adds the primitive to the primitive lists corresponding to those sub-regions. The primitive list building unit also records the positions of the primitives in a pair of histograms which show the distribution of primitives across the render output. Once all primitives for the render output have been sorted into lists, the histograms are outputted to a predictor processor. The predictor processor then determines a set of sub-region sizes to be used when sorting primitives for the next render output to be generated into lists, based on the histograms.