Bullet weight and powder weight are both measured in grains but are separate weights. Since you refer to 165 and 180 grains I figure you are talking about a .30 caliber for an example. So, on the side of a box of factory ammo the weight given is for bullet weight only. Factory ammo will never list powder weight due to so many types of powder.

As far as the relation of bullet weight to powder weight, as bullet weight is increased for a given cartridge powder weight is decreased. Say for a .30-06 you use 62 gr. of H4350 for a 150 gr. bullet yet 58 gr. for a 180 gr. bullet. (Example Only). Lighter bullet, more powder, more velocity. Heavier bullet, less powder, less velocity. Why? Because you are limited by pressure. Both of these loads would produce the same pressure. If you stuffed the same powder for a 150 gr. bullet under a 180 gr. you would be risking excessive pressure.

If you ever start reloading you will see this right away in the manuals. To get a better handle on the idea, go to your local reloading supply and pick up the free manuals from the various powder makers. Or get your hands on a complete manual from one of the bullet makers. Even if you do not plan to reload these manuals will give you a better understanding of your cartridge.