A friend had a lot of extra fluid (like two litres instead of one) - although they couldn't really tell how much until she was in labour and her waters broke. They can't tell how big the baby is so they can't really tell how much fluid is in there either. Lots of guesswork going on!
The only difference it made to my friend was that she was huge and they hassled her about her baby being too big (which she wasn't).
If you have lots of extra fluid its called Polyhydramnios but it doesn't sound as if you do.