Good point. The grain of a bullet is refering to what the bullet weights. Sincea 30 caliber bullet is larger in diameter that 270 bullet which is .277 diameter, the only way to get the same weight, is to make it longer.

Bullets could vary a little in weight if different densities of material were used but as a rule the length is where you will get the gain in weight.
Pick up a good reloading manual if you are interested in ballistics and such. The contain a world of good information.