This may be because the lower moisture content, which is 12.8% for the MC, generates less expansibility and contractility than is necessary to destroy the soil skeleton.

than比较分句省略了形式主语it 和外置的that主语从句。补全的句子为：

This may be because the lower moisture content, which is 12.8% for the MC, generates less expansibility and contractility than it is necessary that it should generate to destroy the soil skeleton.

说明：第一个it为形式主语，真正主语为that从句。that从句中的it为人称代词，指代主句的主语the lower moisture content. 及物动词should generate的宾语为比较对应项，在than分句中必须省略（或理解为than充当了宾语）。to destroy...不定式短语为目的状语。