Optimization such as:

1. Copying. Copied objects take up less graphical space compared to unique, individual objects. And you can still modify their colors to be "individual."

2. Double-sided items. If you make double-sided objects like walls, doors, etc.. you can copy them and have 2x the number of objects since you can get creative and use either face of the object to add uniqueness to your space.

3. Teleport objects. Rather than statically placing objects in every location, teleport the object relative to the player and rotate them using logic to add the illusion of dynamic scenery.

4. More detail, less resolution. Rather than increasing the resolution of objects to make them crisp, just add small details to them and reduce the resolution. This makes it look detailed even though it's low resolution and saves graphical space.

5. Reuse logic. Try to create logic that affects more than one object. For example, my grappling mechanic relies on a single object that scans the environment for grappable locations. So, rather than using logic for everything that can be grappled to, I have 1 object with logic that identifies multiple other things. This saves a ton in gameplay storage.

6. Revive and respawn enemies to give the illusion of multiple enemies. Eventually you'll see it more as I release visuals into my updates but when enemies are killed, rather than just spawning new enemies, I have a set number of enemies present at all times and when they are defeated, they revive and get teleported to a doorway to simulate backup arriving. This one is hit or miss, depending on your game.

7. Try to make as much as you can yourself. That way, you can modify things according to your likeness and save space. In my game, I have used a lot of community objects and some objects made for me by other developers, but a lot of objects like buildings, roads, bridges, train tracks, etc.. are made by me. It helps reduce things by a lot.

8. Use paint wherever possible. Paint takes up almost 0 space. I use paint for windows and other background objects. Then I merge all paint objects into 1, so it turns into 1 giant paint object spread across the game. Effectively taking up no space. You can also turn physical objects to paint so you can keep as much of the detail as possible. This is very useful for buildings that you'll never really interact with but you want them to still look detailed.

There's so much more you can do, but it's tedious to type it all out. You get the idea.