Mооnst@r
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It weighs less because it doesn't include the scenario maps. That's at least one reason. As for the bugs, we need to take a look. Right now, the fifth mission after Ilia is crashing on my game. Although it wasn't happening before. But I can install your mod from the link you provided and check it.
Added 3 minutes later
On your mod, that mission also crashes immediately after it starts.
With the scenarios (which are separate from the mod and placed in the maps folder), it weighs twice as little.
I launched the mission on your mod, made a couple of turns, and it didn't crash.