In this example, "a bit" supplements "too much" in saying that they drank in excess but not greatly. "Too much" is always more than desired but by adding "a bit," people often soften the blow of their remark, or it can be used sarcastically: "You've drank a lot." "Okay, I did drink just a bit too much." "Don't you think the way you yelled at him was a bit much?"

"a bit too much champagne" is meant to sound a bit ironic. It is an embellishment that highlights that they are drunk. In other words, it is a bit of flourish. It is quite common.

First of all, a bit too much champagne sounds like 18th century, upper-class, snooty brits. A more common phrase would be I had a little too much champagne. Second, on to your actual question! You're saying that you had too much champagne. But how much extra champagne did you have? A lot more than you should have had? A little more than you should have had? You're clarifying how much extra champagne you had.

Bit and too much don't have an opposite effect. It means that the person drank too much, but not excessively You could also say he drank way too much - that would show it was excessive. I agree with one of the posters above. We often tend to use it as a bit of a euphemism. If I said, "My friend has had a bit too much to drink", I mean he is really drunk.

一點太多