Just filmed a short clip from the TV onto my iPad. Interesting to note whilst filming the whole thing filled the iPad's screen yet when I played it back the whole thing was reduced to about half the iPad's screen size.

Anybody know why this is?

It's because the iPad makes the film in 16:9 aspect ratio, the standard for widescreen television, although the iPad itself is 4:3, like 'old' t.vs. It fills the screen as it is monitoring, but some of what you're filing is 'beyond the edges'.

Basically, it's so your finished movie will fill a widescreen tele.