Films do tend to be getting better at this, but in the 1980s in particular, filmmakers seemed to have a field day with grenades that were capable of causing the same degree of damage as a decent-sized bomb. In the above clip from Robocop, a regular hand grenade blows an entire house to smithereens, and in James Cameron's Aliens, its use in a vent is again extremely excessive in creating a pretty big fireball. In reality, grenades are designed to cause massive damage in a very confined space; hence the explosion of a real grenade is far smaller than you will typically see in a movie, far away from what in Robocop better resembled an entire house full of propane tanks being detonated with ten-dozen sticks of dynamite.