Let me see if can say this right.
Shallow pots make some begonias happy& some larger pots will retard their growth.If some are given roomy pots they may not bloom as often even with fertilizer as they bloom when pot bound.I have an angelwing begonia a cane type& it does BETTER in large pot than smaller pot.I have another Rex in hanging basket& it does good cause is small sized basket(plus is fiber lined)
your Iron cross is a rizome type.I don't have one,but is on my list as I liked the foliage.I hear the flowers
are worth much to look at& the one I seen they pinched it alot and was under florecent light 14 hours a day.
I have a wax begonia grown inground& they say it grows best pot bound,but the best one I have is the one in ground,the one in pot looks leggy actually.I grow my begonias in african violet mix with peat added& most get AM sun for about 4 hours.I don't fertilze when the temps are below 50(christmas till March)Wax will tolerate more sun actually,but if I give them morning sun....when it gets hot...they will still bloom,but if in full sun would not.
you will really enjoy begonias.I just
my angelwing.Its over 7 ft tall right now.