In general 3D printers are compact and smaller than RP machines. That's ok. But, what's the difference? 3D printers can be used as RP machine too.
All rapid prototyping means is automatically producing a physical part from a cad model. 3D printing is a way to achieve rapid prototyping. There are 2 main methods of rapid prototyping: additive, and subtractive.
A 3D printer is additive- you add materials to an object layer by layer.
Usually, when people talk about a subtractive machine, they are talking about a CNC mill (or lathe), which tend to be extremely large (most are over one ton). You start with all the material there, and you subtract the material that you don't want. This might be what you are thinking of.