The 20" government profile barrel is thin under the handguards, it is only thick from the front sight forward. The thin part is to allow the attachment of a grenade launcher. I believe the thick part is made that way to make the barrel less likely to bend when troops use it for a purpose for which it was not designed ie a pry bar.

I don't think there's any appreciable practical difference in accuracy btw a M4 type barrel and a regular chromelined or chrome moly superlight or hbar 16" barrel. I think most of us end up with m4 type barrels because they are the most readily available. The Colt thin barrels are fairly expensive and there is a bit of a wait for a Bushmaster superlight barrel. J&T has a thin barrel, but I have not seen one, I am considering getting one.