Grain size critically determines both yield and quality in crops. This study systematically investigated two distinct qingke varieties with contrasting grain sizes through integrated dynamic phenotyping, transcriptomics, and weighted gene co-expression network analysis (WGCNA). Key observations indicated that statistically significant differences in grain length and width between the two qingke varieties became apparent from 13 DAF. Transcriptome sequencing (RNA-seq) analysis revealed a peak in the number of differentially expressed genes (DEGs) at 13 days after flowering, indicating that gene expression patterns during this stage might regulate the establishment of grain length and width morphology. These DEGs were likely key regulators of grain dimensions. WGCNA analysis identified a highly correlated brown module enriched with MAPK pathway genes, which showed significant associations with grain size (length, width, thickness) and weight. Functional analysis of the MAPK pathway candidate gene HvPR1 demonstrated that its over-expression in barley significantly increased thousand-grain weight (TGW) while reducing grain hardness (GH), thereby uncovering a previously uncharacterized regulatory role in grain development. The observed reduction in grain hardness in over-expression lines was primarily attributed to decreased lignin and cellulose content. These results provide critical genetic resources and novel mechanistic insights to support precision breeding strategies for qingke and other cereal crops.