Ogryn Ripper Guns need to be big and metallic, as they are designed to withstand their users wielding them as clubs. While the chunkiness fits the proportions of the miniatures, they don't get slimmed down for realistically-proportioned people in other artwork leading to people wielding bricks with magazines as pistols. The lasgun used by the standard human soldiers of the Imperial Guard ranges, depending on depiction, from fairly bulky ◊ to hugely cumbersome ◊.Because of the limitations on humanity's technological capabilities, durability and ease of manufacture and repair are prioritized over aesthetics, and so a lot of Imperial weapons and vehicles are boxy and bulky.Baze Malbus's heavy repeater cannon in Rogue One is a boxy piece of kit ◊.In Blade Runner, Deckard's gun is a mild example, but is still very bulky when compared to the.Also note that it can be collapsed into a slightly more compact form when not in use.
Somewhat justified because it's basically the Swiss-army knife of weapons (it even incorporates a net launcher!). The ZF1 ◊ from The Fifth Element isn't exactly rectangular, but it definitely fits the "large and clumsy" mold.
Justified as he often builds things himself from spare parts.
Starship Troopers.The Verhoeven movie gives us these wonderful gems.Aliens and Avatar, both directed by James Cameron, have some seriously boxy guns.These weapons are used by the Great Asian Alliance soldiers during their (unsuccessful) invasion of Yokohama. The Irregular at Magic High School feature assault rifles shaped like large boxes.The Seburo C-25a is rather on the boxy side though. Most of the guns actually look quite curvy, and some plain looks like a fictional version of FN P90. Averted by the Seburo Arms line of fictional guns used in the shared universe (including Ghost in the Shell) made by Shirow Masamune.Mobile Suit Gundam 00 plays with this trope, occasionally playing it straight, with the likes of the Seravee's GN Bazookas, and even the mecha itself (being that mecha are weaponry, and all), but also completely averts this with the likes of the Ahead and Alvaaron, and falls somewhere in-between with the 00 Raiser, which has a mixture of square weaponry (GN Sword III's gun part) and sleek, pointy things.While most (non-mecha) guns in Macross Frontier look more or less similar to modern weapons, the heavy rifles carried by EX-Gear troops definitely play this trope straight.