Blu-ray stores more data because its laser makes smaller marks
25 GB vs 4.7 GB
A standard single-layer Blu-ray disc holds 25 GB, compared with 4.7 GB on a single-layer DVD. Blu-ray uses a shorter-wavelength blue-violet laser and optics that focus it into a smaller spot. That allows data marks and tracks to be packed more tightly on the disc. The difference is physical: more information fits into nearly the same-sized object.