LightingData for additively loaded scenes baked together is much bigger than for a single scene with same geometry

The game loads game world additively in chunks as separate scenes. Probes and GI need to be baked in these chunks, so prior to this all chunks are loaded in the editor and baked together. This produces LightingData file which is much bigger than if the same geometry were to be baked in a separate scene.

How to reproduce:
1. Open Scenes_U/United_Small scene and bake it. You'll see that LightingData is 11Mb.
2. Open Scenes/Small scene and drag the following scenes into the hierarchy:
> Scenes/Row_1/House7_Indoor_1_1
> Scenes/Row_1/House7_Indoor_1_2
> Scenes/Row_1/House7_Outdoor_1_1
> Scenes/Row_1/House7_Outdoor_1_2
(Attached a screenshot of how the hierarchy should look)
3. Make sure that the level looks exactly as in Step 1 and bake lighting. You'll see that the new LightingData is 18Mb.

In case of 7x7 grid of buildings the difference is ~300Mb vs ~3600Mb.

Attaching Realtime GI UV Charts views for both scenes, they look the same.