There's no active circuitry in a Type A USB cable, just wire connections.
Internally, the cable itself is mechanically clamped to the metal connector shell, and the wires terminated to the connector. The 'box', as you call it, is a housing over-molded onto the back of the connector (in this case, probably using an appearance sleeve with molding plastics injected after.)
This overmold housing serves several functions:
- protects (insulates) the wire-to-connector joints
- provides a strain relief for the cable
- provides a grip to plug/unlug the cable.
The second function is important. Without it, the cable is prone to damage from flexing.
And yes, the connector is bulky, intentionally so. Nevertheless, the cable you show complies with the USB spec for size. It's meant to be robust and easy to handle - no pins to break off, withstands a lot of mating cycles, etc.
That said, in response to observations like yours, smaller USB versions were introduced later - Mini, Micro (used on Android phones) and now Type C.