Re: Bargain buy.
Instructions often tell us to plant in pots only one or two sizes bigger than the existing one.
I used to help out at an organic nurseries a while back, and the lovely man there used to say to do this because they get fresh food and nourishment in new compost each time you repot, makes sense to me.
I always think plants in flower beds are much more independent and can send their roots out looking for what they need, but when confined in pots and tubs they are entirely at our mercy for food. Bit like keeping an animal in a cage all its life.