the Tianocore UI is centered in the display, it's not supposed to be full-screen. If the logo is showing at proper/native resolution, then everything is working as intended.
The display resolution may be set as expected, but the text mode output is being restricted by a PCD in UefiPayloadPkg*.dsc. See https://github.com/benjamindoron/edk2_coreboot-dev/commit/b6fd530b8f552d57e5....
However, I'm not certain how it interacts with https://github.com/MrChromebox/edk2/commit/cd5b08f9aa34a255c3b39c996a0f47609... (or what that commit achieved).