Fermenter volume vs. batch size

Hi everyone,
I'm just about to run my very first batch and while everything seems pretty much in order on paper, I am wondering what is the impact of a largely overdimensioned fermenter on a much smaller batch size: 25 liters vs. 5-6 liters targeted batch. Is that too much air that would oxidize the batch? or will the carbon dioxide produced during fermentation form a protective layer between the batch and the air in the bucket?
Thanks

Should be fine, they used to ferment in open vessels back in the old days.  Generally you want more volume than that though, but I find allowing 8 inches of headspace at least makes blow off requirements no longer necessary. Just dont plan to leave it in there for too long . Once it has completed its activity and cleared up  bottle or keg it.  Then it can age with appropriate seal and minimal airspace.