HD Picture size

I have one Time Warner Cable HD DVR box in the living room with my Samsung Flat HD 32" connected with a HDMI cable. On my patio, on the other side of the wall, I have a 32" Sony WEGA CRT Digital HD connected with SVHS to the same box. In my bedroom, I have another Samsung flat HD, 26", connected with a coaxial cable to the LR box because it's the only connections going to the bedroom(I have wireless transmitters to change the channels on the patio and the bedroom).
The image on the LR Samsung with an HD channel is great. On the patio Sony connected with SVHS, ditto with the large border top and bottom. But on the Samsung flat HD in the bdrm, on coaxial, the picture is much smaller. Takes up just the central part of the screen leaving a large border all the way around. I've tried all the available picture sizes but none give me a full screen HD picture(zoom 1 & 2 crop, 16x9 stretches). What's up? Is the bedroom coaxial connection the issue? Any solutions other than new wiring if that's the issue?