When Captain America goes toe to toe with Iron Man, you better belive it'll affects the rest of the hero community, so a giant cast for Civil War should make sense.
(Here's the full line-up from left-to-right, top-to-bottom: Chris Evans as Captain America, Robert Downey Jr as Iron Man, Scarlett Johannson as Black Widow, Sebastian Stan as Winter Soldier, Anthony Mackie as Falcon, Paul Bettany as Vision, Jeremy Renner as Hawkeye, Don Cheadle as War Machine, Elizabeth Olsen as Scarlet Witch, Paul Rudd as Ant-Man, Chadwick Boseman as Black Panther, Emily VanCamp as Sharon Carter, Daniel Brühl as Baron Zemo, Frank Grillo as Crossbones, William Hurt as General 'Thunderbolt' Ross, and Martin Freeman in an unspecified role).