1 gallon pots & root bound?

Why do yall say it will get root bound? I have seen see of 16 oz party cups from clones to harvest. And 18 gallon rubermaid tubs running hydro with 18 sites(1 gallon per plant and produce 16 oz.. thats with 1 gallons....

Hey guys i usually lurk around here a lot to find some info. on indoor grow and i'm having a doubt on this one. As well as the op i'm using 1 gallon buckets and today i decided to pick them up and look under them (they are a light black that if i put up to the light i can see though) because the growth is ridiculously slow, i mean, they are 4 weeks and not even 4 inches high with only 2 leaves. The thing is other ones are doing fine but are also slow, but not as much as those, maybe have 3 sets of leaves on them.

What i noticed is a giant curled up thin root mass that seems to be wanting to escape out of the drainage holes, the containers are about 10 inches high and the same in diameter (i can also see some on the sides). Is ot normal for a plant not even 4 inches high to have such giant roots ? In outdoor i've seen 6 feet plants that when tucked out of the ground barely had 1 foot worth of roots.

Any suggestions on what to do ? i'm open to getting more soil and containers, are they completely screwed ? they look healthy but at this rate it's gonna take them months to get to a decent height. By the way i'm using 400w hps and no nutes at the moment, just what the soil had in it, nothing special as it wasn't very expensive.

Thanks.