Changed out the soil today and found this in all three. They put so much energy into the roots early on, the tap root is easily twice the mass of the above ground trunk.
My experience as well. I have 4 small seedlings (this is their 2nd season) that I moved into small 4" Rootpouch bags. Will be moving into 1 gal bags this year, crossing my fingers that the bag helped with the issue of the thick tap root. I also have 2 pre-bonsai that I bought, those will probably be like the one you posted above as they have been grown in a container.