Kevin Feige addressed this in an interview with IGN
He explains that while there are a lot of characters who show up, they don't get full story arcs.
"There were a lot of characters in The Winter Soldier, but it felt like a very singular and relatively simple thriller. Civil War follows in that same way," Feige said. "I think that's something [directors Joe and Anthony Russo] pride themselves on, and our screenwriters Chris Markus and Steve McFeely are excellent at giving each character just enough. They're not full arcs for everybody; it's just enough that their presence is felt and important, but that the very clear single story that is being told is being served at all times.They're not full arcs for everybody; it's just enough that their presence is felt and important, but that the very clear single story that is being told is being served at all times."
The film is ultimately about Captain America proving Bucky's innocence. The Civil War is just a plot device to that end. We get lots of guest appearances, but none of them have much effect on the real heart of the story. You'll note that the final battle involves just Cap, Bucky, and Tony.
the final battle involves just Cap, Bucky, and Tony.
Even Anthony Mackie discussed this with MTV recently
That’s what I thought at first, when I first read it, but not at all. It really plays well with the storyline of Cap, and how he’s evolved into his own man, him doing his own thing, everybody coming together to fight for the common good of man.