The cellular expansion of maternal tissues has been suggested as strongly associated to grain volume determination in wheat and is therefore a key process to understand potential grain weight determination. The dynamics of grain growth in seed size-contrasting diploid, tetraploid and hexaploid genotypes of wheat were evaluated to explore the relationships between the final grain weight and: a) grain volume, b) water content, c) epidermal pericarp cell size and d) the rate of cell area (area per cell) increase. The assessment of cell expansion was reinforced by the study of the expression of the XTH5 gene in pericarp tissues, as this gene codifies a cell wall-remodeling agent. Differences in grain weight across ploidies and experiments (greenhouse and field evaluations) were attributable to the grain-filling rate (ANOVA p < 0.05; R2 = 0.91 and R2 = 0.98, respectively), with no significant effect of grain filling duration. Strong correlations were found between final dry matter of grain and both volume (R2 = 0.98; p < 0.01) and stabilized water content (R2 = 0.99; p < 0.01) of grains across genotypes. The epidermal cell area was positively correlated with the final grain volume (R2 = 0.71; p < 0.05) and with the grain-stabilized water content (R2 = 0.72; p < 0.05). The increases in grain volume and grain elongation were accompanied by epidermal cell enlargement during grain growth across the genotypes. Also, at each ploidy, the rate of cell area increase was greater in the heavier grain genotype, which showed faster dry matter accumulation (p < 0.05). The XTH5 gene was expressed during the high-rate water accumulation phase with a significant decrease (p < 0.05) at 320 - 350°Cd after anthesis, shortly before the grain water content leveled off. This work provides new evidence for the relevance of pericarp cell size in the regulation of grain volume and the potential grain weight of wheat.