yes. A 3 or 4 foot plant in flower should have a root mass enough to fill a five gallon bucket. Not completely but pretty good. I have never had a plant over 5 feet so I dont know after that.
Do you mean what happens when the plant's roots outgrow the bucket?...The netpot itself should only be where your very first roots are. I use the small 4inch netpots in my 5 gallon buckets, haven't had problems with it yet.