Beer tastes flat, but is PLENTY carb'd

Ok, I brewed a Mr Beer batch (2 of the same), in 2 of their kegs, and then racked them both into a 6.5 gal carboy. so 4 gallons of beer in a 6.5 carboy, needless to say I had a lot of room.

Would this much space for air cause a flat taste?

No, not typically. There is enough dissolved CO2 in the beer that will come out of solution when you rack that it will create a blanket over the beer. The flat taste is likely from the kit. Those Mr Beer kits aren't really noted for making the best beer.

Think of Mr Beer as a starting point to making quality brews. It will familiarize you with the basics of sanitation and fermenatation but a lot of the work is done for you in the liquid malt they give you. As you advance, you'll add your own hops to unhopped malt and control the level of bitterness and flavour and the maltiness of your beer. If the hobby really has you by the nuts then, you'll advance to all grain and then you'll really be a brew ninja.

If you serve from the carboy, you'll have REAL ale, which is flat. (at atmospheric pressure there is some CO2 in the beer, but not much)

Otherwise, you'll be priming & bottling or putting it back into the Mr. Beers to carbonate.