Morbidly A Beast:
Just eat until you couldn’t possibly eat anymore, eventually eating big will come so naturally you won’t feel satifsifyed by what would be seen by most as a large meal.
i thought i would add to this, because "eat until you couldn't possibly eat anymore" might be interpreted differently by different people... this is what made a real difference for me when i was early in my gaining (180-250 lbs).
i would eat until i felt full. *really* full. and then i would push past this, and eat some more - stopping well before i thought i might be sick. it didn't really matter what i ate (they were just regular foods - just more volume), but it was the "beyond stuffed" feeling that was stretching my stomach.
it takes time. you can't do it every meal, every day. if you just do it a few times a week, you will start to notice that you won't feel as full as quickly. that's because your stomach is getting stretched out, its capacity is larger.
when you get a big ol' tummy you'll just feel like eating more, more often.