My inner geek has been rummaging again - the whole great/grateful spelling thing has been driving me bonkers. And, actually, had one thought about it, the answer was quite simple...
Grateful is a derivative of Gratitude. Hence the "grat..." I.e. Full of Gratitude. Not full of greatness. Duh. I'd never really considered the actually meaning of either word. Had I, it might have been easier to figure out. Though they sound phonetically similar, they are not the same thing...
So now you know (in case you cared). This has been a public service announcement.
