Unity large level size in build

Every level in my project weights about 50mbs and i do not know why. If i duplicate an identical scene it will keep adding +50 mb on top even if the whole scene is a prefab. Any way to check what is causing this? A lot of previous forums suggested to change lighting settings but nothing helped, prefabs are not broken. Has anyone faced a similar issue? The editor log doest help much, just says that the levels are 98% of the build weight.