As for the "4+ 1 port" you said, it may mean that there are five ports in the TL-SG 1005D, of which four ports are used for equipment connection and 1 port is used for power input. This design is common in switches and aims to provide more network connection options.
Regarding the detailed information of Mercury SG 105D, I didn't find a direct comparison with Pulian TL-SG 1005D, but I can infer that the design concept, internal components and functions of the two products may be different, which may be the reason for the volume difference. Usually, the increase in volume may be related to more cooling space, more interfaces or more complicated internal design.
Generally speaking, the volume increase of PL TL-SG 1005D may be due to its more functions and/or more complicated internal structure to meet higher performance requirements.