The Bigger They Are...

The Bigger They Are... is a Windows/Xbox 360 achievement and PS3 trophy earned by killing all 5 super mutant behemoths in Fallout 3.

Locations of behemoths

 * GNR Building Plaza
 * West of Jury Street Metro in the train yards.
 * Evergreen Mills locked in a cage
 * In the Capitol Building rotunda.
 * In between Takoma Park and Takoma Industrial.

GNR behemoth

 * To find the GNR behemoth just follow the main quest without skipping any of it and you'll come across it. (WARNING: talking to Doctor Li or entering the Jefferson Memorial or Vault 112 before you fight this behemoth will cause the Brotherhood of Steel soldier's corpse to not appear near the fountain outside of the building and thus not letting you take the Fat Man and mini nukes from his body.)

Jury Street Metro behemoth

 * The Jury Street Metro behemoth will spawn if you approach the cart cages, one of which contains a teddy bear. The cage can be found inside an overturned railway car, west of the Jury Street Metro, or east along the tracks leading out of Evergreen Mills. You may have to wait a moment for the behemoth to appear, but you'll see it start charging at you from over the hills a few moments later. If you leave and return after approaching the cages, the behemoth might be standing next to them. If the behemoth does not appear, fast traveling to Jury Street Metro should force the behemoth to spawn there.
 * If any of the caravan merchants (Crow, Crazy Wolfgang, etc.) are nearby when you open the cages, they or any of their entourage may be killed by the behemoth. They may also help you kill the behemoth.

Evergreen Mills behemoth

 * The Evergreen Mills behemoth is caged in its own electrified slaver pen so you can kill it from the cliff heads above or shooting through the fence surrounding it. (The behemoth can escape if the generator by the gate of the cage is destroyed, which can be sniped from an out cropping of the cliffs south of Evergreen Mills marked by a billboard.) Any followers (Ex. Dogmeat) that spot the behemoth can free it from its cage and engage it.

Capitol Building behemoth

 * The easiest way to find a behemoth in the capitol building is to enter through the west entrance.
 * It is possible that the Capitol Building behemoth will be killed by a Talon Company merc-- you find the behemoth and some other super mutants engaged in battle with Talon mercs, one of which has a Fat Man.

Takoma Industrial behemoth

 * The Takoma Industrial behemoth is found by heading to the left of a battle between Talon mercs and super mutants to a truck with a merc and a switch to a mini nuke artillery strike on top of it. If you can see the Behemoth and Takoma Motor is on your left, the truck is behind you. The Takoma Industrial artillery strike can be fired numerous times, but the switch may break. However, it can be repaired with a high enough Repair skill level, as well as a level 33 or higher Science level. Once it has been repaired, it cannot break again.

Behind the scenes
The name of this achievement/trophy is derived from the English idiom "The bigger they are, the harder they fall."

Bugs

 * The Jury Street Metro behemoth may not appear at all. If this occurs, try waiting for at least 5 in-game hours. If that doesn't make it appear, then restart your console and try again until it appears.
 * While facing the Takoma Industrial behemoth, if you call in artillery while the behemoth walks. The behemoth may teleport to the top of your head.
 * The Jury Street Metro behemoth sometimes spawns before you open the cage or take the teddy bear.