Cargo airplanes are freighters in the sky. They are designed for one specific purpose: to carry cargo, hence their name. Because of this, the bodies are designed wide to carry as much freight as possible. Being designed as wide as possible, special considerations must be given to the construction. Bear in mind luxury is not on the construction list since the intended use is for cargo only.
The Beluga (also known as the A300-600ST Super Transporter) was developed to carry complete sections of Airbus aircraft from different production sites around Europe to the final assembly lines in Toulouse or Hamburg. Its main-deck cargo volume is greater than the C5 Galaxy, the An-124 or the C-17 airlifters.