The UI doesn't work on your real resolution, but on some kind of virtual one. Run this command to see your UI resolution:
/run DEFAULT_CHAT_FRAME:AddMessage( ( GetScreenWidth() * UIParent:GetEffectiveScale() ).."x"..( GetScreenHeight() * UIParent:GetEffectiveScale() ) );
For me on 2560x1440 with an UI scale of 1.0 it prints 1365.333...x768.
And if you run the following code that creates a frame that's 1280x720 you'll see that it nearly fills the whole screen.
Code:
frame = CreateFrame("Frame", nil, UIParent)
frame:SetBackdrop({ bgFile = "Interface/Tooltips/UI-Tooltip-Background", edgeSize = 8 })
frame:SetWidth(1280)
frame:SetHeight(720)
frame:SetPoint("LEFT")
frame:SetPoint("TOP", UIParent, "TOP", 0, 0)