Renowned tipster on Weibo suggests that the iPhone 18 Pro will feature a significant upgrade to its main camera, which he claims is now 'confirmed'.
This enhancement will result in a physically larger camera module compared to its predecessor, leading to an approximate 2mm increase in the thickness of the camera bump.

Reports have long indicated that the iPhone 18 Pro and iPhone 18 Pro Max may incorporate a main camera with a variable aperture. This feature could account for the increase in size. Additionally, there is speculation that a larger sensor may be implemented, further contributing to the increased dimensions.
The launch of the iPhone 18 Pro and Pro Max is anticipated for September, alongside the new iPhone Ultra. In contrast, the iPhone 18 and iPhone 18e, as well as the potential iPhone Air 2, are rumored to be released in the following spring.
Source (in Chinese)