I think some people fail to realize just how large the casts for these type of films typically are.
I mean, all of these superhero films are ensemble films to begin with, often having a sizeable amount of core and supporting characters. Beyond that, in a 2 and a half movie with a large scope, there is more than ample room for dozens of other side characters to make appearances, have lines, and even appear in more than one scene to play a small role in the story or have an impact.
Think about a film like The Dark Knight and the amount of characters that appeared.
Bruce/Batmam
The Joker
Alfred
Rachel
Harvey Dent
Gordon
Lucius
Mayor
Loeb
Maroni
Gambol
The Chechen
Scarecrow
Wuertz
Ramirez
The other fat cop who let the Joker overpower him
Lau
Gordon's wife
Gordon's son
Annoying SWAT truck passenger
Crazy Joker goon with Rachel's name tag
I mean, this is just off the top of my head, but all of those characters either played major roles, small roles, or made brief appearances. Most of the 20+ characters I named were actually essential in advancing the story along.
My point is that it is FAR from unnatural for a film like this to have a huge cast, and so far, this cast doesn't really seem to be all that huge. Frankly , I'm surprised anyone's even bugging about it, since Cyborg's role has all but been officially confirmed as a cameo and Mamoa's is likely the same -- a brief appearance.