Twitter is rolling out a new feature for iOS and Android mobile apps that allows you to see larger and taller images in your timeline rather than cropped 16:9 images. Twitter is also killing the ‘open for a surprise’ tweets that are commonly posted that show a cropped picture and then a silly, or adult-themed, one when expanded when expanded. The feature should be rolled out within the next day or so, and it should be rolling out to all Android and iOS users within a few days.
Source: https://www.bleepingcomputer.com/news/technology/twitter-kills-open-for-a-surprise-tweets-with-new-mobile-feature/