Changes in the agrarian structure are among the indicators of Polish agriculture’s adaptation to competition with the EU agriculture and therefore require a thorough examination. The purpose of this paper is to determine the scale, direction and level of diversification of Polish agricultural farm size structures in 2010–2020 by typological groups of voivodeships. The research was carried out on the basis of Statistics Poland (GUS) data on the number of farms by size groups for the years 2010 and 2020. The following farm size groups were analysed: up to 2 ha of agricultural land (AL), 2–5 ha, 5–10 ha, 10–20 ha, 20–50 ha and over 50 ha. Based on fuzzy classification, four typological groups were distinguished, comprising voivodeships with similar farm size structure. Then, changes in the structure of the typological groups and changes in the number of farms in absolute terms were presented. The structure of the typological groups changed only marginally, while changes in the number of farms in absolute terms were very significant. In all groups, a decrease in the total number of farms was observed, notably farms of up to 10 ha in size, as well as an increase in the number of farms larger than 20 ha, with the rate and scale of those changes varying regionally. The largest decrease in the number of farms was recorded for voivodeships with the most fragmented agrarian structure.