A BFG is chiefly defined by, well, its incredible bigness. The BFG is a visual metaphor for power, and therefore has a lot of uses as a trope. The BFG may have the recoil effect of flying backwards after firing. The gun must be oversized, not just "a little larger than usual". A pistol as large as a weasel isn't that impressive. A Hand Cannon, on the other hand, is a subtrope of BFG where the pistol is at least as large as an infant.
When in the possession of a little girl it probably overlaps with Small Girl, Big Gun.
Not to be confused with Roald Dahl's book and character The BFG (Big Friendly Giant), TNA Wrestling's show Bound for Glory, Big, Friendly Dog, the starship battle game Battlefleet Gothic, "Bang!" Flag Gun, or another type of gun.Although the BFG could be thought of as compensating for the aversion of the latter.