Big Team matches have anywhere from 12 to 16 players on the map at any given time. The maps are much larger than most of the Infinity Slayer maps and almost always incorporate vehicular combat into the match.
For the most part, the concepts of the gameplay are the same as Infinity Slayer. You want to kill the other team to score points and gain progress towards ordnance. You will run, jump, sneak, shoot and punch your way to victory. The tools that you will use to do this may vary however.
Make sure that you brush up on your vehicles and how to counter them. Be sure to mix up your load outs as appropriate and try to play a little bit slower. While there are more people on the map, the size of the map means that running into the middle is just going to get you killed. Taking sections of the map as a team is far more important in Big Team than going for a multikill since the number of enemies that can attack you will be greater while the number of enemies in close proximity for you to kill will be much lower.
Because of the greater distances, I highly recommend the Light Rifle. When zoomed in, the weapon can kill with torso shots in five shots which is the minimum number of shots for any of the other precision weapons when they hit the head. This means that any time you can keep the weapon zoomed, you have an advantage and this is doubly so if you are well concealed on a large map. Further, it means that on the largest maps where you may only barely be able to see an enemy even when zoomed, you don't have to get a head shot to kill them.
Your secondary weapon is most likely to be the plasma pistol on this playlist. While the magnum might be nice for closer range battles, the number of vehicles running around on the map make the Plasma Pistol a fantastic weapon of choice. It instantly gives you an anti-vehicular weapon every time you respawn.
Grenades are a little bit fuzzier than either of those weapons but the plasma grenade has a slight advantage. While the Frag grenade may appear to be very useful because of how often you end up in firefights with precision weapons, the fact that almost all of these maps will take place on large stretches of dirt and grass minimizes how well you can control the bounce. Since your Frag Grenades are likely to act like a Plasma Grenade, you might as well take one. It will increase the damage of your grenades and allow you to stick them to moving vehicles.
There are a few useful armor abilities. Jet Pack can be useful for getting yourself above a vehicle for boarding it. On larger vehicles like Wraiths or Scorpions, directly above them is their biggest blind spot. Active Camo is a favorite since there's a lot of space and people aren't likely to notice the scrambling effect. This allows you to snipe in safety (which is common on the big maps) and it will allow you to close in on vehicles (which are common on these big maps) without being seen since they don't frequently look at their radar.
While Regneration is generally useful, on the larger maps it isn't quite as effect because you aren't generally that close to your teammates.
Bringing extra ammo can be pretty helpful since your accuracy will decrease at the longer ranges and give you more time before you need to find another weapon if you have a particularly good spawn. If you can pick up a specialization, there are a few that give you mods that increase your skills while driving that are particularly helpful in Big Team.