Comperative adjectives
I know that the sentence below is correct
" I'm busier this week than I was last week."
But is it wrong if I say
"I'm busier this week than last week."

Question

Answer

No, that is not wrong, at least in casual conversation. The second sentence implies the "I was" portion, even if it is not said. In formal writing, it should be included.

Answer

Mike's comment is correct. Just FYI, in conversation I would most likely say, "this week is busier than last week."