I started 5g AG batches with a 7.5g kettle.
I never had a boil over, but I had to keep a very close eye on it.
Then when the first kettle died, I got myself a 10g kettle.
Since then, I've had several boil overs.
One day I may learn that I have to keep an eye on the brew as it is coming up to the boil in a 10g kettle, but I wouldn't like to bet on it.