The thing is, that player should take a step back and think...the DM and all the other players disagree with me. Who's wrong? I've played in games with other DMs of far less experience and have sometimes totally disagreed with their calls, but at worst I've only ever asked them to consider how I saw it. If they disagreed I would say fine, you're the DM, that's that. Often the DM and the other players would say "Sure that makes sense" and rule in my favor. But when they didn't, we just let it go and kept playing.
This is why I avoid telling the players if I change something without them knowing it, as in this case. Better not to mention it, and then when they encounter this larger pudding and say: "But...but...but the last one we faced didn't split in two!", you can always just say: "Well maybe that one was not fully developed, or was of a slightly different species or maybe this larger one is a mutated form of black pudding. You might want to spend some of that gold consulting a sage on the issue."

