Third-generation (3G) snacks are obtained by an indirect extrusion process using lower temperatures and pressures than the traditional method, which favors the preservation of thermosensitive compounds and their storage due to the smaller pellet size. The aim of this study was to know how different compression ratios (1:1, 2:1, and 3:1) and different rice particle sizes used affect the sectional expansion index (SEI). Due to their similar nutritional composition, rice flour (RF) and rice grits (RG), both prepared with the same water content (25% moisture), were used as raw materials. A single-screw laboratory extruder (Kompakt extruder KE 19/25; Brabender, Duisburg, Germany) was used to obtain 3G pellets, which were dried at room temperature (25 ºC). The pellet expansion process was carried out using a microwave oven (MW), obtaining the expansion kinetics for both types of snacks. The expansion kinetics showed that both samples began to expand one minute after microwave heating was initiated, with the 3:1 compression ratio offering the greatest expansion. It was also observed that the particle size of RF (60-630 μm) was smaller than that of RG (105-1096 μm). The results of the SEI measurement exposed that snacks made with RF had a higher SEI than those made with RG. It was concluded that the 3:1 compression ratio favors higher expansion due to the higher pressure exerted at the nozzle outlet with respect to the other compression ratios. However, no significant differences (p > 0.05) were found in the SEI of the samples related to particle size.