User blog comment:Some Argonian Named Kesch/Stormcloak Rebellion: Is Anyone Really Right?/@comment-5700881-20131111004831/@comment-17114085-20131112115103

You can't really take what you see in-game to heart. Whiterun isn't that small, neither is all of Skyrim for that matter. I'd guess it would be around 3-4 times larger than what we saw in-game. Thus making siege weapons/magic the only way to breach the Walls/gates.

PS.: The Gates weren't unlocked because of gameplay/bethesda lazyness. Realisticly speacking the Doors would have been bared.