생각했던 것 보다 is similar to 보다, but adding 했던 relates the comparison to a past action/state – in this case, your previous expectations:
만든 음식이 생각했던 것 보다 더 맛있었어요.
The food I made tasted better than I thought (it would).
이 영화가 생각했던 것 보다 더 길었어요.
This movie is longer than I thought.
한국에서 미국까지 비행기 표가 생각했던 것 보다 더 비싸요.
My plane ticket from Korea to the USA costs more than I thought.
The structure can also take different ‘thought’ verbs and intensifiers:
그것이 생각했던 것 보다 훨씬 쉬워요.
It’s much easier than I thought.
그의 여자친구가 상상했던 것보다 더 매력적 이었어요.
His girlfriend was more attractive than I imagined.
여행이 기대했던 것 보다 오래 걸렸어요.
The journey took longer (more time) than I expected.
날씨가 예상했던 것 보다 훨씬 나빴어요.
The weather was much worse than predicted.