Farmland size increase (FSI), which is an increase in operational size and consolidation of fragmented croplands, is considered an effective scheme to improve agricultural productivity and reduce fertilizer use with environmental protection benefits. But FSI involving road extension can enhance rill formation on road surfaces, and increase road erosion and associated nutrient losses in a watershed. These effects have not been experimentally quantified yet. Thus, we determined the changes in road surface rill erosion and associated nutrient losses in response to FSI before (2017) and after (2018), in an intensive sugarcane production area within Nala watershed, in southern subtropics of China. We divided the watershed into three sub-watersheds (lower (S1), left (S2) and right upper (S3) regions), and surveyed three categories of roads: (i) old roads (existing roads before 2017), (ii) new roads (constructed in 2018), and (iii) old-new roads (repaired and extended old roads in 2018). The road surface rill erosion and associated nutrient losses were much higher in the lower region (S1) than the upper regions (S1 and S2) of the watershed due to variations in field density (number of farms per unit area) and traffic intensity. Field density reduced by 53 % while traffic intensity increased by 41 % after FSI. Following FSI, annual road surface rill erosion, nitrogen, phosphorus and organic carbon losses increased by 88 %, 79 %, 67 %, and 46 %, respectively. Road surface rill erosion had a significant inverse relationship with field density (R² = 0.535, P < 0.05) but positively correlated with traffic intensity (R² = 0.728, P < 0.01). The accelerated N and P losses offset the decrease in fertilizer use by 5–22 % due to FSI. Our results highlight the significance of including soil erosion in FSI scheme for environmental protection. Accelerated road erosion following FSI should be minimized by integrated soil and watershed management practices.