Root Balls

I read somewhere that when you plant a (bare root) tree that you should use the same soil that came out of the hole to fill it . The reasoning was that if you use nice soil when the roots hit the edge of the hole they would treat it as if it were a planter/pot and just fill the "good soil" with roots until it is root bound . I'm wondering if the same principle applies to the veggies I've grown here . I dig a hole a good bit larger than the root ball/soil in the small pots I use for seedlings and fill it with a mixture of manure and soil from the hole when I transplant . We don't have much topsoil here , and in my inexperience when I first started my garden I think I let a lot of it wash away (the garden is on a slope) . I'm amending that now by tilling straw into the soil after using it as a thick mulch to retard runoff and keep weeds down . But the soil is still mostly in pretty poor condition so I still dig the holes and fill with an "enriched" mixture . I just got to wondering , thinking back on the size of the root balls in past years . Seems like they never get any bigger than the original hole ...