The only part where you'll lose space is because you have to import different objects with different shadow maps/ or it's a lot easier to clamp some scenes together so you'll have for a region of the map one big scene. One solution would be made for example one rock with one tree shadow on it and copy that rock whatever you have the shadows of trees visible for the non shaded rock you just use the normal version. So you'll only have one extra model and that`s it but you have to be inventive. The good part of the shadow technique is that it uses reale small shadow maps only a few kb depending on the obj and complexity of the shadow and you won't need to import other textures, the same texture remains an all objects, white shadow maps or not, like a ingame texture for ex, only with one s map over it. Let me give you a estimated space consumption- My interior scene is 4mb in total and take note it`s with animated shadows so the room with just a simple shadow map could be just 200kb or less.