Vegetables/Fruit
Stuff like cabbage is practically water

But really it comes down to your metabolism and how often you want to do it. Having one enormous meal once a week isn't going to do much. I had to have a very large meal 3 times a week before I started noticing any significant weight gain.

But I also don't much care for the taste of junk food

A quick swig of water is a big help when you've got a mouthful of food and are having trouble getting it down.

If I'm eating a variety of food around the halfway mark I'll get something cool and non filling like a bowl of ice cream or a salad to give my stomach time to settle.

Most of eating in excess is based around overcoming your mental limiters but beyond that is there much room for improvement? Stomachs aren't really an elastic organ, they just kind of fold in and fold out to contain food. I don't know how body parts react to being stretched out but I'd imagine it would be difficult to do with a stomach considering the limited amount of time food actually remains inside. There is also the fact that the stomach needs a little wiggle room so it can do it's job.

Most if not all competitive eaters say they've had large appetites since childhood, so are they just predisposed to having a larger stomach capacity?

You can see how fast you can drink a gallon of milk
I can drink the first half in 60 seconds but I can't get the second half down until another 9 minutes later...

I'm not very good with cold things
I can drink a gallon of milk in around 15 minutes so I'll just wait for the shakes to melt

I don't think an exact volume measurement would be possible without an x-ray or something.

Like you said water starts passing very quickly while food can sit around for quite a while depending on what it is.

I usually just go by weight to make things simple. A big meal after an hour and a half of eating usually comes out to 10lbs. An all day stuffing starts pushing towards 20...