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Soil aggregates swell when infiltrated by water, and their size can increase in two ways. First, aggregates can attach to one another with water acting as a bridge (i.e., adsorption), and water can enter the aggregate pores (i.e., swelling). This is defined as the “adsorption-swelling” effect. Second, clay can fill the pores of macroaggregates and microaggregates (i.e., filling) when adsorption occurs. This is defined as the “adsorption-filling” effect. However, the size range of aggregates affected by these effects and the extent of their influence on aggregate swelling are still unclear. Therefore, different initial size fractions (5 ∼ 2, 2 ∼ 1, 1 ∼ 0.5, 0.5 ∼ 0.25 and 0.25 ∼ 0.053 mm) of soil aggregates from the black soil zone of Northeast China were studied. The size range of swollen aggregates, the “adsorption-swelling” rate (V) of initial size fractions, and the “adsorption-filling” rate (E) of size fractions < 0.053 mm were measured and calculated in three experimental treatments that involved the following procedures: i) wet-sieving of each initial size fraction in deionized water (WS); ii) wet-sieving of each initial size fraction of air-dried aggregates after they were soaked in absolute ethanol (WSas); and iii) the size fraction < 0.053 mm air-dried aggregates were mixed with each initial size fraction of air-dried aggregates in absolute ethanol and then wet-sieved (WSaf). The results were as follows: i) the size fraction 2 ∼ 0.053 mm were swollen. ii) V decreased exponentially with decreasing initial particle size, with a maximum value of 32.30% at a size fraction of 2 ∼ 1 mm; and iii) the “adsorption-filling” effect of size fraction < 0.053 mm was obvious in the size fraction < 2 mm swelling aggregates with a maximum of 29.54%. The “adsorption-swelling” and “adsorption-filling” effects had greater impacts on soils with high contents of the size fractions 2 ∼ 1 and < 0.053 mm. This study provides a theoretical basis for understanding the swelling mechanisms of soil aggregates.