Non-replicative herpes simplex virus type 1 (nrHSV-1) vectors are promising delivery vehicles for gene therapy due to their large DNA payload capacity and ability to infect a broad range of cell types. However, the genomic deletions made to generate such nrHSV-1 vectors can result in undersized genomes that trigger genomic instability—including rearrangements and size extensions—compromising their therapeutic potential. This study investigates the stabilization of undersized nrHSV-1 vectors through the insertion of stuffer DNA segments. We assess genomic stability, productivity, toxicity, and transgene expression in vitro and in vivo. Our findings demonstrate that nrHSV-1 can accommodate variations in genome size up to 5–6% and highlight the importance of maintaining a genome size close to that of the wild-type HSV-1 for enhanced genomic stability and sustained transgene expression without adverse effects. This strategy offers a promising approach for optimizing nrHSV-1 vectors for clinical applications.