Going back through the standard ship types in the OTU I am beginning to think that a Squad of Marines is 8, (Patrol Cruiser) A platoon 35 (Kinnunir), and a Company just over 100. That would give you 2 x4 man fire teams per squad. 4 Squads per Platoon, plus Platoon Leader, Platoon Sergeant and Runner/Clerk/Commo Specialist. If you run an HQ element for a Company as 10 (Commander, 1SG, 8 various specialists) then you have 115 for a Company.
That puts them at about what the starships seem to be designed around. Using those numbers the Use of the Kinunir class is the fore runner to a class similar to the Rodger Young from the book Star Ship Troopers. (Deploying a Platoon as drop trops with a pinace for recovery.) Accordingly in MTU there are follow on 1250 ton DE that are primarily designed for Platoon sized drops. (Since no Navy ever has enough light combatants they are of course also equipped to be used as DEs.) then a Destroyer Sized ship, older first introduced at TL14, for deploying a Company of Marines. (Again multitasked to also serve as a Destroyer.) the Cruisers tend to carry a Company Plus of Marines and using these as the heavy end of Piracy suppression it makes sense. Destroyers and Destroyer Escorts are the Navy's Primary Anti Piracy ships so they should carry Marines for Boarding actions, destruction of Pirate bases etc. A TL15 Platoon in Battledress inserted from orbit (and in MTU battledress has Grav modules as well as leg power) can act on a typical planet well out of proportion to its size vs. TL12 or less troops, especially in the Mid tech range. Their firepower tends to be less than a Battle Tank but in T20 the idea of them being defended like an MBT makes sense in that role. to kill Armor they can use FGMPs but against higher tech armor they would be forced to rely on fire support from their orbiting ship and/or Assault shuttle/recovery boat. A larger unit, Starship Troopers has a Platoon Size of 48, makes sense in many cases but that would greatly increase Transport sizes and/or require special ships instead of ships that can multi-task. A Larger number of Companies can make up a Batalion and give greater flexibility for actual deployments. It would make sense, given the size of the Imperium and the Elite nature of the Imperial Marines to have smaller more flexible units that today's USMC.
Just a thought.