Sheikh Rabee and many others from the 'Ulema, in commenting on this verse*, explain the reason for speaking about Allah without knowledge being greater than shirk is due to the fact that shirk results and emanates from speaking about Allah without knowledge.
* Say: "(But) the things that my Lord has indeed forbidden are Al-Fawahish (great evil sins, every kind of unlawful sexual intercourse, etc.) whether committed openly or secretly, sins (of all kinds), unrighteous oppression, joining partners (in worship) with Allah for which He has given no authority, and saying things about Allah of which you have no knowledge." (Al-A'raf: 33)
Subhanak Allaahuma wa bihamdika ash-hadu anlaa illaaha illa anta astaghfiruka wa atubu ilayk
If I said anything correct, then it is from Allaah (subhanahu wa taa'ala), and if I erred, then that is from me and shaytan.