Troop transport. Perhaps the Terran Heavy Infantry are all in battle dress and grav belts and carry a variety of weapons so they're very lean on the organic heavy support, instead choosing their battlefields and "raiding" more than conquering, serving as skirmishers and raiders. A 300-ton displacement ship might be capable of deploying a hundred of such soldiers with room for the quarters, stores, spares, and so on. It would also give them the tactical and strategic agility to be deployed separately from the larger combat armored infantry formations to where they're needed.
Cargo transports. In my Crusade Across the Stars campaign, smaller military cargo transports are a thing. While everyone understands economy of scale resulting in increasing savings for the larger the ship, the militaries typically have "highliner" (the lovechild between a Herbert's "Dune" Guild highliner and a modern container vessel - it carries fully-loaded planetary landers on a huge "spine" and stops just long enough in-orbit disgorge landers and pick them up while concurrently things like crew changes and refuelling occurs, then it jumps out), "transport vessel" (more like a straight container vessel today), and a "frontier transport." The frontier transport would be a smaller cargo transport. This is fulfill a niche transport need which requires a ship that is more responsive than the larger cargo transport to move things, not from a depot to the operations areas, but between operations areas. They also make up about 40% of the logistics fleets - because shipyards that can make the huge (and cost-effective) transports are limited, a grinding large war requires more cargo capacity than they can produce, so many smaller shipyards make frontier transports to aid in the war effort. The frontier transport is designed to be rugged, field-repairable, and contains many features that are lower-tech than standard because they can be field maintained. The ships make do with as little microelectronics as possible because older, mechanical systems can be field repaired and spare parts fashioned more easily. That such systems have a much higher maintenance burden and require larger crews is accepted - sophisticated microelectronics can be in short supply in the deep frontier, but crewmembers can always be found.