Although usually translated as “God is Great”, أكبر or Akbar actually means greater (comparison). So what is he Greater than? Imam Al-Sadiq (as) explains..

And from him, from Yaqoub Bin Yazeed, from Marwak Bin Ubeyd, from Jami’e Bin Amro, from the one who reported it,
(The narrator says) Abu Abdullah (as) said to me: ‘What is it that ‘الله أكبر ’ Allah is the Greater than?’ So I said, ‘No, by Allah, I do not know except that I see Him as Greater than everything’. So the Imam (as) said: ‘And He existed, and then there were things, so how was He greater than it (everything before there was a thing in existence)?’ I said, ‘So what thing is Allah Greater than?’ He (as) said: ‘He is Greater than from what can be described (beyond description)’.

Source: Al Mahaasin, Al-Barqy, Volume 2, Book 1, Hadith #1158