Full-size pickups differ from compact pickups mainly in their packaging, available configurations, and balance of passenger and load-carrying capability. A full-size model generally offers a larger vehicle platform and may provide more cab, bed, or towing options. A compact aims to provide useful pickup functions in a smaller footprint. Neither category has one universal bed size or payload rating.
The most useful comparison is between two actual vehicles. Body construction, driven-wheel layout, cabin arrangement, and equipment can vary independently of the size label. Understanding those differences helps explain why one truck works better for a particular load or parking space, without assuming the larger vehicle is always the better design.