Invasion maps are large out of necessity. There are twelve players total, and they need the necessary space to be treated as 6 individual teams. The space that a map must yield is twice that of a normal map, because a full Game Play occurs in one half of the map for Tier 1 and then the other half of the map for Tiers 2 and 3.
From an earlier lesson we saw that the map is just one map, not two maps. But the map must be designed with dimensions as though it were two separate maps. Tier 1 must be designed to allow a full scale invasion landing force to establish its presence and breach the barrier to the enemy strong hold. Tier 2 begins Game Play around the enemy strong hold. Tier 1 must support 12 players up to but not beyond the barrier. Tier 2 and Tier 3 again must support twelve players as well as a neutral extraction point for the data core at the end of the Round.
Smaller maps tend to create confined areas or small play spaces, creating the feeling that players are on top of each other continuously. This has the effect of making for very fast paced game play, but it also leads to a lot more deaths and less time for strategies to unfold. At the other extreme, distances that are too large bore players as they spend too much time traveling and not enough time engaging the enemy.
Distance is a Defenders' friend - it allows them time to setup a defense. Distance also gives value to Forward Positions and make Bro Spawning more critical for Invaders. These are valuable Invasion Game Elements that a forger will want to emphasize by utilizing distances through out the map.
The following list demonstrates Tier 1 distances that the Play List Maps present to give some idea of what Bungie had in mind as reasonable distances for an Invasion map.
Boneyard Alpha Spawn to Forward Position: 60 steps; to Objective: 30 steps
Boneyard Bravo Spawn to Forward Position: 60 steps; to Objective: 30 steps
Spire Alpha Spawn to Forward Position: 50 steps; to Objective: 30 steps
Spire Bravo Spawn to Forward Position: 40 steps; to Objective: 30 steps
Breakpoint Alpha Spawn to Forward Position: 40 steps; to Objective: 15 steps
Breakpoint Bravo Spawn to Forward Position: 25 steps; to Objective: 15 steps
Summary
An Invasion map is larger out of necessity, accommodating 12 players as 6 teams twice over.
Smaller maps are too fast to setup strategies, and larger maps are boring to walk around.
Distance is a Defender's friend and makes Bro Spawning the Forward Position valuable.
Average Tier 1 distances are roughly 50 steps from Spawn to Forward Position and an additional 25 steps to the Objective.