Re: Shapeshifter : The Hawk-whale strategy
I would allow it.
It's a classic metamorph trick. Beast Boy, the Vision and Hank Pym do this sort of thing it all the time.
There are rules for increasing size/mass during a move through, but I'm not sure where they are in the 6E books
In 5E I believe they are covered in the Combat Handbook (p137), which I beleive are reprinted from the Champions main book. I think it was also covered in the Ultimate Metamorph...
You have to make sure he has his powers done correctly to allow for it though (the multiform transformation needs to be fast enough, for example, or he needs to fly high enough).
Of course, he's going to probably end up taking a lot of damage doing this, so I hope the whale form is tough enough to take it.