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Social Interaction
Interaction is one place where your hit points, number of spells or sword skills don't necessarily matter, so having long interaction scenes helps. Characters can get involved in epic politics, and still be 1st level schmoes. "Fights" or other adventuring activities will be at once fewer and faster for small parties, leaving plenty of room for talking. Yes, I know, revolutionary talk. TALKING. I'll even let (nay, encourage) the few characters talk amongst themselves so that they build up their own relationships. This makes the game more immersive and satisfying, regardless of how many Orcs you kill. It's also why I like to pump the setting up with strong recurring NPCs, and why I have problems with picaresque adventure structures. If the characters are always on the move, how can they build relationships with NPCs? Strong NPC presences give the PCs a reason to talk. They'll visit the same old contacts, love to hate rival adventurers who poke at them, and want to defeat that villain you've been building up for many sessions. Mentors, family members, familiar merchants... If the players have interesting characters to talk to (and who can act as doorways to mystery and adventure), they can get involved in the world, set their own goals, and feel that their story is moving forward, even if their hit points aren't going down.
The One Opponent
I also like to take the focus away from group encounters and attrition strategies and instead build up a single opponent (or equal number of opponents). Not so much monsters then, but the one rival/enemy that the PC wants to stop, defeat or annoy. He or she can be of equal power, or could remain unattainable until the right moment, driving the PCs to better themselves. A small or weak party will have trouble with large and repeated encounters, so instead emphasize problem solving until the time comes for a climactic fight. They can follow clues, defeat the villain's traps, perhaps have a small fight along the way (do whatever you need to keep your players interested according to their styles of play, of course), until they are ready for a final confrontation. Even when dealing with monsters, necessarily weak because of one's level, a single monster or monster species will be more effective than a dungeon zoo.
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Tighter Focus
Whether your game depends on a balanced mix of classes or not, characters will have a certain range of abilities which excludes others. In a smaller team, you may not have access to healing, long-range blasting, stealth or even combat. Embrace that. Tailoring adventures to the small skill set not only makes sense but makes the players feel their characters are valuable. Thieves get involved in heists, warriors in battles, priests in exorcisms, and so on. Even in a Supers game, where power levels tend to be higher, Nightwing solves crimes on the street level, while Superman fights would-be world conquerors and Dr. Fate is expected to repel supernatural threats. A tighter skill focus can also be used to justify multiple characters evolving in the same class. Two warriors who are part of an army unit will share adventures. Two thieves working for the same guild infiltrate a rich merchant's home. Even when there are two classes represented, have the thief get help from his warrior mercenary friend, or the wizard be called in to investigate magical goings on at his cleric friend's temple. Again, why would you fill an adventure with traps if no one can disarm them? Or with battle encounters if there is no fighter on hand?
In short, small or weak parties aren't an obstacle, they're an opportunity to tell a different story.
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