Minecraft for the PC is 8 times the surface area of the earth. They could have gone a lot bigger if technical limitations were the problem, you don't need all that amazing of a PC to run minecraft smoothly. Xbox doesn't have much RAM, but a computer with 8 times as much and the same processing speed would have no problem running it, so it should at least get like 1 times the size of the earth.
I doubt it has to do with being lazy. They did a good job with bringing this to xbox, problem is the xbox can't handle everything that pc can. I think it was either because of limitations and to avoid the game freezing consoles or just being overall laggy. It could change as time goes on with updates as they figure out ways to make it work. But other than that the map size doesn't bother me too much as i've been playing a while and heven't run into the invisible wall, you notice it the most if you constantly worry about it. I'm just glad I can actually play it now, due to the fact that my pc is crap and can't run minecraft without freezing.