I've been thinking about this one for a bit before responding. I think it's a big miscommunication from both sides and the guy is probably not controlling her food intake or hates fat people, etc. I'm basing this on a few things from the original post:
1. When the guy sees the popcorn, he visibly frowns and gives her what she feels is an odd look.
If I had said to someone that I don't want any snacks, and then they come back with what clearly looks like popcorn for two, I'd be slightly miffed myself because I would feel like my preference had been ignored and I'm being manipulated into eating something I didn't want. The OP even points out that she got the larger size in case he wanted to share. He may have felt she didn't respect his decision not to have any but now that she had purchased it, he was obligated. His strange look may have been about that more than he was miffed she got something for herself. Also, my experience with guys is that they all tend to eat popcorn in big messy handfuls instead of 1-2 at a time like women do so the description of him shoveling it in doesn't make me think he was trying to keep it to himself just that he lacks...finesse. I learned early on that if I wanted my share of uncrushed popcorn, I need my own container.
2. and he gives her the drink back but holds on to the popcorn.
This might be his attempt at chivalry -- the guy holds the greasy popcorn for the lady so she doesn't have to juggle two containers. From the description, they are in the hallway at this point and this is where she should have insisted to get her snack back rather than wait until they are in the theater. If she didn't reach for it or ask for it, he may not have thought it was OK to force it back to her.
3. He “lets” her take another handful, but then puts the popcorn down, by his feet away from her.
"let's her" is a prejudicial way of saying it so I'm going to think that she simply took another handful while he was still holding it and there was no begging on her part and his acquiescence. Since she was putting the popcorn in her lap, he may have been tired of holding the bucket and didn't think she wanted any more. When she asked for some more, he could have simply given her the container to be sure but did she reach for the bag to take it or did she just stick her hand in? He may have been waiting for her to simply take it since he clearly doesn't want it. It's also a dark place and he's probably trying to concentrate on the movie so holding firmly might have been because he didn't realize she was taking the container from him and thought it would get knocked over.
The third time she indicates she would like some of her own popcorn again, she thinks he audibly tutts at her. Embarrassed, this is the last time she reaches for her popcorn.
Again did she reach for the bag to take it or just put her hand in to get some popcorn? If he keeps holding it out to her and all she does is take a couple pieces each time, he may have been exasperated at continually being the keeper of the bag and being interrupted. Lots of people like to "get lost in" an action movie like Batman, he sounds annoyed that she seems to be more interested in the popcorn and keeps "pulling him out." I'm a bit befuddled myself that through this whole movie date, she's just so fixated on that popcorn.
At the end, why did she need to just leave it, why didn't she just respond to him that she was taking the rest home? So I vote not a red flag, just two people who didn't clearly communicate their desires.