A/V receivers usually have multiple components such as an amplifier, tuner, and preamplifier, which all require space. To fit everything in, receivers need to be relatively large.
Amplifier
The amplifier section is the most space-consuming part of a receiver, as it requires significant amounts of heatsinking to prevent overheating. The amplifier section is responsible for amplifying the incoming audio signals, so the larger the receiver, the more powerful the amplifier can be.
Tuner
The tuner section is responsible for receiving radio signals and converting them into audio signals. Tuners require space for their RF circuitry, as well as the various switches and buttons used to select stations.
Preamplifier
The preamplifier section is responsible for amplifying the low-level audio signals coming from the amplifier, and providing tone control and other processing. Preamplifiers also require space for their circuitry, as well as the input and output jacks used to connect to other components.
Other Features
In addition to the main components listed above, A/V receivers often include additional features such as a phono stage, surround sound processing, and HDMI switching. These features all require additional circuitry, which adds to the size of the receiver.
Conclusion
Overall, the large size of audio receivers is due to the multiple components and features that they need to accommodate. While there are some compact receivers available, the larger models offer more power and features, making them the preferred choice for many audio enthusiasts.