“That’s a Bit Much” Means “That’s Unreasonable”

“That’s a bit much” is common in informal Australian and British English. It does not simply measure an amount; it says that a request, behaviour or situation is excessive, unreasonable or difficult to accept. If a colleague asks you near knock-off time to redo an entire report that day, you might say, “That’s a bit much. Can we finish it tomorrow instead?” The words a bit soften the criticism without removing the objection. It is more conversational and indirect than “That’s unreasonable”, although a sharp tone can still make it confrontational. The phrase is also understood in American English and is not uniquely Australian. For a literal quantity, be specific: “That’s a bit too much food.”

https://dictionary.cambridge.org/dictionary/english/bit-much
https://www.collinsdictionary.com/dictionary/english/a-bit-much