It depends on the resolution of the file you're watching and what form of Airplay you're using.
If the movie is recorded in widescreen or HD, if you use Airplay, it should play in full screen on the TV. If it's 4:3, like old SD tvs, it will play in a square box.
On the other hand, if you use Airplay mirroring
, because the iPad screen is 4:3, and you're just copying pixel by pixel, it will always appear as a square box on the TV.
Your laptop has a widescreen like the TV, so, since they're the same shape, they will display in full screen. You can also not mirror and have two independent screens, in which case each will show in it's native, full resolution. You can't do two independent screens on an iPad--you can either send just the video, or the entire screen mirrored.
Because you say you are watching an HD broadcast, and it's still letterboxed, I think you are using mirroring. That will transmit the black bars above and below the video on the iPad to the TV, and then, because the TV is showing a square imagine, it'll have black bars on the left and right, so it looks like the video is the right shape for the TV, just shrunk.
Airplay mirroring is a systemwide setting that apps can't control, whereas Airplay has to be coded into each app by the developer. Based on a quick google search, it looks like Comcast has not programmed Airplay support into the app set, so mirroring is your only option. The only way to make it full screen that I know of is to use an iPhone 5, which has a widescreen, and use airplay mirroring off of it.
edit: you can tell if you are using Airplay or Airplay mirroring by quitting the current application. If the home screen appears on the TV, you're mirroring. If it goes back to the Apple TV menu, you're using just plain Airplay.--
I use Linux, OS X, iOS and Windows. Let the OS wars die.