What Twitter is currently testing on the iOS version of Twitter is testing a screen layout that fills the entire timeline screen. As a result, photos, GIFs, and videos can be viewed on a larger screen than now.
This is a layout that allows the media to be viewed in a larger size on the smartphone screen by moving the user icon on the left side of the image up until now. This layout has been tested with some iOS users and it is yet to be officially implemented.