The major benefit of consoles is that they are a fixed hardware platform, so developers can do a lot of optimizations to the code that they could never do on a PC. On the PC they need to make sure that the code works with AMD and nVidia GPUs more or less equally well, but on the Xbox they need to only target the AMD GPU in the console.
A game console also is generally a single task device. It's a little less true with the 360 and PS3 especially after everyone finished creating their me-too answer to the Wii, but generally speaking when you run a game, the whole of the console's resources are given over to running that game. On a PC you have the AV software, the significantly higher overhead of the OS, and whatever else might be running on the system.
PC versions of a game might have options like anti-aliasing, and various detail levels, all of which are fixed on the Xbox.
Finally, and this is just barely scratching the surface, it doesn't matter how big the TV is, the maximum resolution output of the Xbox is 1080p or 1920x1080. The bigger the TV screen the further apart the individual pixels are and/or the bigger the individual pixels are. With PC monitors you might have higher resolutions, and higher resolutions means you have to draw more pixels, etc.