There are varieties of carrots that have been bred for extra sugar, but you won't find them in your typical grocery store. Most of the grocery store carrots have been bred for color, shape, production and storage.
In grocery stores that sell loose carrots, you can sometimes find 'winter carrots'. They're quite large (nearly 2" / 5cm across). These are carrots that have been left in the ground over the winter -- they are typically much sweeter than carrots that are taken out of the ground in warmer weather. I have seen larger carrots at the latin markets and restaurant supply stores near me, but I've never done side-by-side taste tests to confirm if these are sweeter than standard grocery store carrots.
There are also seed companies (eg, Baker Creek, Burpee, Johnny's, Territorial, etc.) who sell varieties bred specifically for their sugar content, much like what's being done with corn. If you grow your own, you can seek these out, or you can check to see if any of the farmer's markets near you carry them. Much like heirloom tomatoes, they're often a bit stranger in color and shape than what you get in the grocery stores -- they might be short and stubby, or have a dramatic taper that isn't useful to make 'baby carrots')