Allahu akbar means God is great. Allahu means God. Akbar is a form of the adjective great and is used for an emphatic. You can say it's "great," "greater," or "very great." But there's no "our" in the phrase and it isn't meant to compare one god with another. In Allahu akbar, akbar doesn't mean "greater" than another.
Kabir=big, akbar=bigger. It's a comparative in the grammatical sense.