Speaking of trying to express the scope of Rogue Trader - one of the techniques I'll definitely be using in the game is what I call the Microcosm / Macrocosm divide.
I first studied this is college. The idea is that often in fiction or media, there will be a single conflict which is supposed to symbolize a greater struggle going on in the background. This isn't too hard to understand. We all understand that at the end of Return of the Jedi, the real struggle that is going on is in the throne room of the Death Star. And if Luke loses his battle with the Dark Side, then everyone else on his side also loses.
We see this in a lot of Bond flicks. James Bond goes in, confronts the bad guy in the end, while a raging battle of MI-6 agents and minions takes place behind them. When Bond sweeps in and does this, the audience immediately understands that this final conflict is what really matters. The big battle scene is just a backdrop. Heck, we even see this in Shakespeare. The big battles in the plays Richard III and in Macbeth are determined by a small conflict on the field with just a few players.
I suppose for roleplaying, we could have conflicts that are more "realistic", where the main characters are only a small part of the battle. However - how much fun would that be?
I've used this idea to great success in a lot of my other campaigns. Back in the day, I ran a sci-fi military game for my friends at Minions of the Monster Master. The main characters were members of an elite fighting division called Alpha Wolf Pack. Anyhow, I always explained to my players that in large scale battles, their success or failure would determine the fate of the entire army. No pressure.
So, for example, I might stage a battle with the player characters and an elite squad of opponents. Even though the PCs will never fight the rest of the combatants on the battlefield, the conflict with the elite squad basically determines the outcome of the entire conflict.
I had been using this idea of the macrocosm / microcosm for a while. However, down in Minion Land, I really refined that idea. I created NPCs in the party's retinue who essentially represented portions of their entire platoon or division. For example, I might have a sniper NPC who represents all of the scouts in the party's division. Or I might have a tank commander who represents all of the tanks that are supporting the party's division. I explained carefully to the players at the outset that if something happened to one of those NPCs, their division would symbolically share the same fate. So, if you lose the sniper in a battle, all of your scouts also die in the battle. If you lose your tank commander, you also lose all of your tanks. This was very effective, particularly once the PCs became fairly powerful and difficult to harm. Suddenly, the game wasn't just about keeping yourself alive. It became about bringing your boys home.