I find on discussions with my friend who =really= knows the story, that
Pippin and Merry got another chance at ent-draughts while on their way North
and home after the War of the Ring. When they speak with Bilbo at Rivendell,
they tell him that they may decide to rival Bullroarer in height, as he is
rivaling the Old Took in age.
Before that, at the Field of Cormallen, Sam swears that they have grown
"three inches" since he last saw them.
So it would seem that the growth was gradual, perhaps three inches for each
stay with the Ents, so that when the final 6th inch was entered, Pippin would
indeed at 4 feet 6 inches overtop Bullroarer's 4 ' 5 ".
However, I am still worried about the average-sized Hobbit handling a
=full-sized cart horse= (not riding it, rather driving it as it pulls a sort
of omnibus-cart full of halflings) in one of Sylvia Hunnewell's pictures for
the BreeMoot program book.
Settling the important questions of our day,