Fighters come in fleet scale and 'mech scale. Fleet scale fighters are usually about 0.5" long, and are meant to represent squadrons in space fleet battles. 'Mech scale ones are exactly what they sound like, fighters that are in roughly the same scale as the normal ground stuff. Those are intended to be used during aerial dogfights, smaller space fights, and ground attack missions.
If you wish, there is nothing at all stopping you from using fleet-scale minis with 'mechs, or 'mech scale minis in fleet fights. They're just there to give you options.
DropShips also come in two scales, fleet scale and map scale. Fleet scale ones are just like the fighters, though they're not to the same exact scale. Some are only slightly larger than some larger fighters, but they are usually big enough that you can tell the DropShips are supposed to be bigger. Battletech spacecraft minis do not use a hard scale for everything, otherwise it'd be impossible for fighters to be bigger than specks of metallic dust, or for battleships to be smaller than lunch boxes.
Map-scale DropShips have the same purpose as 'mech scale fighters. However, since any DropShip would be absolutely enormous if in scale to a 'mech mini(try to imagine assembling, painting, and transporting a Union that was bigger than a beach ball), they're instead scaled to fit on the mapsheet so they can represent landed ships. Again, the size isn't to scale, but big enough that you know the DropShip is the biggest thing on the map.
