They're all MASSIVE... Why?
Basically this. Harry, Mouse, and Mister are all huge, why do you think JB did that?
The obvious answer would be something like: "Harry and his buddies are the aegis for humanity, and their size symbolises their giant status/power compared to those they protect."
It's easier to hide behind a big shield than a small one. And Harry takes it upon himself to be that shield, even when his friends don't want it.
I suppose it also fits Harry's character. He's a straightforward guy - he sees problems and smashes them. The most "obvious" power is in size, so he's big and so are his buddies. We see tons of hyper-dangerous beings that are way smaller than Harry (because size isn't the only way to have power) but for Harry, the simple, obvious, and LOUD path is usually plan #1.
There are variations on these themes, but this (I think) pretty much sums up the obvious answers.
Am I missing something more subtle or important?
P.S. Yes, I'm including Mister in the protection crew. I feel he has a part to play (beyond detecting Spirit Harry).