Just a few thoughts to this that somehow just spontaneously occurred to me - no idea why just now, have read this over half a day ago
From a botanical point of view, I would not suspect any influence.
When topping, the auxin distribution changes and the plant is deprived of the highest shoot, which would grow the strongest with normal auxin distribution.
If you shorten the roots, it has no effect on the hormones that I know of. If there are any, I strongly suspect that they are very different from the above reaction.
All you are doing is reducing the amount of water, nutrients and oxygen that the plant can take up/utilize.
One gardening tip I know is that when transplanting plants, you should compensate for the resulting root loss by cutting back above the roots so that the root mass returns to match the leaf mass and transpiration area.
My guess would be that you're just putting the plant under unnecessary stress and thereby limiting its growth in the short term.