Huge means retention
Kids of any age can have slow "transit" or decide to forgo a stool when the feeling arises.
If a kid creates 5oz/day & empties only 4 or skips, the end of the colon
can baloon out become less effective when empting occurs. What u describe is common. I have the radiologists clean out about 1/yr with a special enema
from the 2 dozen i rx /yr when agressive home care doesn't help. Talk with your ped.