What is the "Heavies" mode in Halo multiplayer?

In the Master Chief Collection, this mode is currently a part of the action sack playlist in at least Reach. I presume that the mode is a variant of the big team battle with a higher amount of vehicles and/or increased vehicle spawns, but can't say for sure. The player health seems identical (Elite slayer has more resistant shields).

According to my research from Halo Waypoint forum, you are right "Heavies" mode is a variant of BTB Heavy but for action sack (4vs4). So you will play with lots of vehicles as Tanks/Wraiths/Banshees ... and lots of heavy weapons as Sniper rifle/Laser/Rocket...