Carboys are slightly smaller than fermentation buckets. They're also notoriously slippery and heavy when full, so you'll definitely want carrier straps. I've used car boys instead of beer bottling buckets before when all my buckets were spoken for. It didn't make as big a batch, and as I mentioned, it was a little bit more difficult to carry, but the tapered neck reduced any unused space and made a nice channel for oxygen to escape through the airlock.  It's usually better for fermentation rather than for the bottling process. I would advise against using it for bottling because there's no way for you to attach a spigot, and things can get very messy when you are trying to keep the tubing through the small neck without touching the bottom.