sure. i have 3 gal going in a 6.5 gal bucket sitting right behind me. for a primary, it doesn't matter how much headspace you have, as long as you have enough (~1.5 gal). it's when keeping a beer in a vessel for a long time after fermentation ceases that you want as little headspace as possible, in my case i'll transfer this beer to a 3 gal carboy for aging after 3-4 weeks in the bucket.