I do about equal amounts of wine with raw fruits and concentrates I buy from a supplier. I can control the size of the batch much more easily with raw fruits. Most concentrates are intended to make a specific volume (e.g., 5 gallons).
My last concentrate batch was a Gamay. One 46-ounce can makes 5 gallons. I followed all directions appropriate, first fermenting in a food-grade bucket, etc. etc. then racking to a carboy, which happens to be 6 gallons.
Because of the gases produced during primary fermentation, I am not concerned with the several inches of headspace between the top of the must and either a towel or lid over the bucket, but after the 5-gallon batch goes to the 6-gallon carboy, there's a fair amount of headspace.
I left it in the secondary for 2 months at 5 gallons. Only then did I get worried about headspace causing off flavors, so I topped off to about 6 gallons reducing headspace to just an inch or so in the neck. ...Not thinking at the time that I had just diluted my wine by a gallon.
Real question is: how much headspace is too much for wines in secondary fermentation or conditioning when it's not producing enough gas to help prevent off flavors? Do you think my gamay would have been fine if I hadn't topped off?
Secondary: sweet dessert mead
Conditioning: black raspberry port #1, black raspberry port #2; annual apple wines (2); Concord wine
Bottled and being imbibed: Santa's Magic Potion (seasonal holiday ale); gamay beaujolais #1, standard port, blueberry dessert wine, ice wine
Bottled and Aging: dry mead, sack mead
On deck: gamay beaujolais #2, cherry wine, peach wine; Trappist Dubbel; English strong ale