During fermentation some airspace in the fermenter is acceptable because of the foaming, and because carbon dioxide is forming and is protecting the contents. A carboy that has airspace in it over the wine is asking for disaster. Oxygen will not only brown and spoil your wine, it will eventually turn to vinegar. Bacteria need oxygen and air to live, therefore it is common practice to keep oxygen and air out of your wine to protect it. Well its not always that easy to find ways to fill up your carboys without diluting the flavors of your wine.
Diluted wine is almost as big of a mistake as oxygen.
However with proper planning, a home winemaker has several options, and will be able to deal with any topping up issues that come along. On the first racking Rack to a 6 gallon container. This will leave some airspace, but generally the wine is still fermenting some, and the carbon dioxide will blanket the wine. This is the first racking stage and the wine will only be in the 6 gallon carboy for a couple of weeks, or until ALL the fermentation activity stops. Generally you can come close to filling the 6 gallon carboy, depending on how much juice or must you started with.