That third night he found his way more quickly than before. He was walking so fast he knew he was making more noise than was wise, but he didn't meet anyone.
—J.K. Rowling, Harry Potter and the Sorcerer's Stone
Why isn't there any subject before 'than'? Usually 'subject + verb' or just nouns comes after 'more than~', as in "Harsh treatment by her mother-in-law was more than she could bear.", and "We prize liberty more than life."
But here, there's no subject and an adjective 'wise' came after 'was'. I don't understand the grammatical structure here, thus I'm also having difficulty understanding what this sentence means.