In Zechariah 4:7, the "great mountain" represents the formidable opposition to the construction of God's temple. In apocalyptic language, mountains often symbolize established kingdoms or religious systems. To Zerubbabel, this was the political and social resistance of the surrounding nations.
When Jesus told His disciples they could say to "this mountain" (likely pointing at the Temple Mount) to be cast into the sea (Matthew 21:21), He was utilizing Zechariah's imagery. The "mountain" was the apostate Old Covenant religious system that stood as a barrier to the New Creation. Its removal was not a literal geological feat but a covenantal judgment.
By AD 70, the "mountain" of the old temple was leveled to a plain. The Roman armies were the providential tools, but the power was the Spirit. With the old mountain removed, Christ, the true Capstone, was fully revealed as the sovereign head of the spiritual temple that now fills the whole earth.