Starting small, and transplanting into gradually larger containers will allow the root ball to grow more dense as you go.
Say you pop a seed in a seed starter or rockwool, once the roots have taken over that media, you repot into a jiffy pot- or 16 oz beer cup with drainage cuts works just as well- and the roots have new boundaries within which to grow as densely as possible. Dense roots make healthy, strong plants. Once the new media is overrun with roots, and they have circled the container a bit, you can move it into a larger pot, generally 8 to 10 gallons at most- then on up into the 50's and 100's if you get there.
Popping a seed in an 8 gallon container for example will result in a stringy, sparse system of roots whereby a strong gust of wind or not so gentle handling could probably rip the plant away from it's roots.
And as Hiboy said, you don't want to be be watering and nutrifying 8 gallons of soil that houses a seedling whose totality encompasses the size of a jar of peanutbutter
