I think you're misinterpreting. "make maps bigger to compensate for increased speed" doesn't mean "bottleneck player traversal routes so you can only go certain ways faster," it means "preserve deliberate pauses in map traversal routes while still accounting for improved locomotion options." Let's take Midship, since it's probably the easiest example. At the start of a match in Midship, players deliberately spawn within their base, and cannot jump from the opening on the second level of their spawn directly onto the center dish. This creates a perimeter-based, cyclical rush at the start of most matches as players take the ramps to their immediate left or right into the center halls before running onto center dish. However, if you were to increase movement speed and jump height, as well as allow for additional movement options that exponentially increase the distance you can travel in a single bound, you might be able to hop from spawn straight to top mid's dish, thus giving you a better chance at obtaining the Sword. This directly conflicts with the intended map flow, and suddenly the majority of the map becomes obsolete as players make direct vectors from spawn to top mid. There are countless other examples of intentional movement / objective restriction or "look, don't touch" landmarks in Halo maps. When 343 is making the maps bigger to account for improved movement options, this is what they're talking about - Midship isn't getting bigger to "level out" the time to travel distance due to increased speed, and we've seen evidence of this with people Thrusting out of spawn to get to alcoves and halls more quickly. However, the center dish being inaccessible without traveling to either second-level "mid hall" is a deliberate design choice, and as such it's being preserved with the increased scale.
This is also why major complaints regarding tools like Jetpack arose, what with the improved vertical movement breaking core "look, don't touch" design choices on maps that were generally designed to account for just a single jump.