The game's map files are baked to be memory mapped to a fixed address, which is part of how they achieve such fast load times (the most time consuming part is loading assets like textures, etc, which aren't fixed to a specific address and may not be sequentially laid out on disc). So a weapon definition in a map file is laid out exactly how it will appear once loaded into memory. So there are pointer-sized fields within the map files. Some of the data structures might have required a different (ie, greater) alignment for a 64-bit platform. This would also be reflected in the map file too.

So everything on disc that is directly memory bound will have to be 64-bit. For Halo, this is a lot of data, but I doubt it would result in anything more than each map file growing by a MB or two in size.

There are some data structure optimizations they can do for release builds to shave off a few kilobytes (eg, remove some pointers which are only used in debug builds). In fact, they did it for Halo2. However, I think they stopped doing them so map files would be binary compatible with debug and release builds.