It wasn't that much smaller--Romney won Texas 57% to 41% for Obama, while Cruz beat his Democratic opponent 56.6% to 40.5%. Cruz and Mitt were running against different opponents, Mitt's being a president who had become very unpopular among much of the electorate in Texas. I doubt if Cruz's opponent, a former legislator, generated as much visceral opposition.
In 2016, Cruz has won nine states and has tens of thousands of motivated volunteers eagerly working for him.