Its all about the amount of content can be filled into the space. High definition requires lots of data space hence the newer optical media, Blu-Ray.

Games, programmers can store more cinematic scene, details on the games, also, optional effects such as demolishing the surroundings, wind effect, lightings and so forth, all requires space to store. Thats why more advance games on PC requires tonnes of space.

Aside from that, movies will not make much difference. DVD movie will be similar if what you meant is the movie alone. Comparing details, BD can store much more amount of detail and also, can be watched on bigger screen without being all jaggy or pixelated. Also, sound takes a lot of space. Surround sound especially.

And one more thing. More special contents also on BD movies!

I leave the rest for the others to add.

Just my 2 cents. Hope this helps.