Namely through increasing the size of the display along with the device's screen-to-body ratio (to 6.41" and by about 2% respectively) by slightly shrinking the bottom "chin" bezel and using a taller, 19.5:9 AMOLED panel with more heavily curved corners & significantly smaller "teardrop" shaped "notch".
This is in addition to the 6T's other major changes which includes increasing the size of the battery, removing the 3.5 mm audio jack, and providing a new optical in-display fingerprint sensor.
[7] The OnePlus 6T comes with a new 6.41-inch FHD+ AMOLED display & taller 19.5:9 aspect ratio (compared to the 6's shorter 6.28-inch, 19:9 panel) and increased screen-to-body ratio (by about 2%, bringing it from almost 84% to around 86%) via the phone's slightly smaller bottom bezel (aka "chin") and the new display's more heavily curved corners & significantly smaller "teardrop" shaped "notch" (for housing the front facing RGB camera & sensors).
[11] Like its predecessors, the OnePlus 6T was praised for its long battery life and performance from an efficient Android build, all while still remaining relatively affordable for a flagship device.
[12][13] The lack of a headphone jack was criticized, as well as the removal of the notification LED, while some reviewers also noted that there is still no IP67 or IP68 protection from water and dust.