Originally Posted by Majuub

Well, the first time he said he wasn't sure he could beat him, he said that to Piccolo, but the second time he said that, there wasn't a shade of doubt, he just said he could have beaten Fat Buu nothing else.

Vegeta said that Goku SHOULD be able to obliterate Chibi Buu at full power. In my translation, Goku said he's been trying. Maybe he meant that he's been trying to reach full power, maybe he meant that he's been trying to obliterate him.