I have a Surface Pro with Windows 8.1 and a PC with Win7 SP1 both fully updated including optional updates (RDP8.1 client for Win7, verified in about box).
Still, when I connect to my Surface from my PC everything is enlarged.
My PC has two monitors: primary 1920x1080 and secondary 1680x1050. I've tried opening connection on each one of them as well as on both (multi monitor). I've also attempted to use windowed mode with lower resolutions.
Surface is set to default DPI settings (slider moved to Larger and "Let me choose one scaling" is unchecked). Also, when I'm connected via RDP I cannot move the slider ("Some display settings can't be changed from a remote session") though I can click "Let me choose..." but that requires a sign-out. Another oddity is that my remote desktop client does not have a "Smart sizing" window menu item.