Whilst aviation technology is not always at the forefront of developments there are significant differences in quality and reliability with the car industry.
HUD more often uses vector graphics; raster is used in some synthetic vision applications. Aviation has to cope with a much wider range of ambient light conditions and meet stringent integrity requirements – no hazardous misleading information (10-9?). These requirements more often require dual inputs, processing, and monitoring, which increase the capacity of processing systems, but not necessarily size.
The most significant size issue is usually the optical / projection system. Modern ‘glass’ combiners use holographic techniques which enable larger fields of view, which in turn require larger display mounts.
The optical size depends on display position – distance from the eye. Military systems can be very small, and ‘Google Glass’ concepts would be feasible if head position with reference to the real world was not required – e.g. flight path vector, true horizon, which may not be the case in cars.