Lack of roots will stunt development, yes. Like trees, the size of the plant above ground is roughly equivalent to the size of the root mass below ground. So a tiny root system will mean a small plant. (This is the premise of bonsai, restricting the growth of actual trees by keeping their root mass small.)

The question is, why such a small root mass? Soil or hydro? I'm a soil guy personally. If soil, what kind? Pot size? (Going from 2-gallon pots to 4-gallon pots produced MUCH larger plants for me, but transplanting during flowering isn't a great idea.) This far into flowering, I doubt there's a whole lot you can do, but whatever the problem is, obviously it'd be good to figure it out before the next crop.