EDITED OP on magicman's request
I rightclicked a bucket with 10 liters of water on an empty copper pot.
It filled the copper pot to 5 liters, and 5 liters remained in the bucket.
When I put the pot over the fire, the blue bar reads 100%, with 5 liters of water.
I then boiled 4 items of food in it, bringing it's water down to 3 liters.
I then rightcliked the bucket that still had 5 liters remaining in it, on the pot, to top it off to full again.
It took all 5 remaining liters from the bucket filling the copper pot to 3 + 5 = 8 liters of water.
I did not put the pot on the fire to see if the blue bar now reads 160%, cause I don't know if that would have caused server issues from overflow values or whatever.
Now the reply below this states that a copper pot is supposed to hold 10 liters, accoding to the wiki.
So either that is not true, and filling it to 8 liters is a bug, or
if what the wiki states is true (which is not always the case, mind you), there are two bugs instead:
the mechanism of filling a copper pot from a bucket is bugged, not putting in the full 10 liters, and the blue bar on the copper pot is bugged. showing 5 liters as 100%