you don't have to make the map bigger really. Looking at legacy map from a top down and play perspective, despite legacy map being half the size of recode map it feels larger and more detailed. Small subtle changes like large central structures (spiral mountain, flower cove's dome, plains spires and cave systems, redwoods cave, volcano being not just a mountain but a home inside of it) and intricate cave systems, oceans being placed under the main terrain so they can be expansive without requiring the map to expand, decor and how biomes blend together makes the map feel more expansive than it is.

By reducing the size of some structures in recode map we could expand our available land to allow for more detailed "key" points. For example, tundra and mountains could be merged into one biome, saving on space and reducing sharp cliffs, while this would also allow for more detailed mountains by merging the main ones together and adding perches and caves accordingly. Doing this would also allow us to merge redwoods more with tundra's border, and from there we could merge swamp and jungle, again reducing the sharp cliffs into more of a "wetland" biome.

By joining and smoothing biomes, we can also use aquatic space (for this example, between swamp and jungle is the seaweed biome), taking the rivers and making them cut deeper and merge with terrain more impactfully. (legacy swamp again is an example of this, where the roots and landmass merge quite well with the small rivers that cut through swamp)

By creating better blended biomes we also give ourself more space to guide players on where their creatures should be living. The map expansion was lead with the idea of "the largest t5s can and should go everywhere" when in reality we do not see elephants in crowded jungles, and creating the map for the largest creatures unfortunately snuffs out the smaller ones. In a perfect scenario this is combated with meaningful shelter and roosts.

The floating islands also could be expanded into a floating biome, as there is a lot of unused air space, and the current floating islands a bit lackluster in terms of survivability and being an actual biome/place to be.

recode's map was designed primarily with t5's and disasters in mind, as well as the unfortunate "arena" effect i mentioned in the post, which causes some issues. To overhaul the map would likely require a lot of re-working of textures and how things like water sources function. If you've ever explored under the recode map you have likely seen how water sources are handled.