TikTok movie/tv show clips

Hey guys, I just started posting on a TikTok account using entertaining clips from movies and tv shows I enjoy, but I’m stuck on how some people post these in full screen good quality ? I’ve managed to now get the videos I want from YouTube onto my gallery, and have put them into the correct aspect ratio for TikTok using CapCut, but it still only fits the clip into half the screen and has thick black bars on the top and bottom of the video itself, any way to get these videos so they cover the full screen ?