That is a little problematic as the ability to change form into another shape with a fixed size comes from a race with a fixed size as well, yet titanic increases it.
Which form is the 'real' one that increases when they are all technically the real one? You chose which?
Also, if, say, a Dragon with Deceivingly Innocent Form at 1st level becomes Titanic and grows can it increase the DIF instead of the dragon form?