This is so basic to most of you, but when I get on the Hi Def source of my Samsung A630, the screen gets smaller on some channells. Why is that?

I assume you're talking about pillarboxes, i.e., the black bars on the side of the picture when displaying 4:3 content. This is normal behavior. It's done to preserve the aspect ratio of the material you're watching. Depending on if you're watching an HD channel or an SD channel, the bars may be added by the upstream provider, or by your TV.

Certain programs will end up in what's referred to as a "windowbox", where they're aired in SD, but letterboxed to 16:9 or so, plus pillarboxed to keep the aspect ratio. This even happens sometimes on HD channels, particularly for syndicated content. On SD channels, your TV should have a "Zoom" control of some kind to let you crop off the letterboxing and make the image fill the screen. Of course, this is not true HD - it's 640x480 or 720x480 with the upper and lower segments chopped off, cutting it down to 360 lines, then blown up to fit. This won't have the same resolution or image quality as HD programming, but it does fill the screen. If this happens on an HD channel, you may or may not be able to zoom the image - many TVs won't zoom on HD resolutions. Try it and see what happens, but don't be surprised if it doesn't work.