You're not the only one with this idea. It has been brought up on multiple occasions in the past. However, there are a few problems with the proposal:
Call of Duty is based around close quarter combat; nonstop action, which is part of
Call of Duty's popularity. If Call of Duty were to expand the maps to the size of Battlefield 3's, the action would diminish and gameplay would slow down.
Now before you say "then add vehicles", think about it. Vehicles are what separate Battlefield and Call of Duty. If vehicles were added to Call of Duty then it would be very similar to Battlefield but with a different titles. Both games aspire to be unique. It's the reason people buy games.
Also, larger maps mean more graphical details. More graphical details mean less FPS, which is the wrong direction the developers want to go. They want to keep a buttery, smooth 60FPS.
If Call of Duty ever happens to use Battlefield sized maps, don't expect it until the next generation of consoles are released. Until then, you will have to play Battlefield if you wish to play on larger maps than what Call of Duty currently offers.
- TopFighter