Maybe a couple of things

A diet often means eating better, not just less. Eating better will usually mean more energy.

You got used to carrying a certain load, then lightened it,so of course felt more energy. I think you could get the same energy from exercising more, without losing weight. It is sort of the ratio between what you regularly demanded of your body and what you are asking right now.

I suspect that calories per 100ml would be the better measure, as I think we feel full more from volume than from mass.

Having said that, I cannot really imagine anything denser in calories than what you are making.

Some other thoughts though:

- Faster really is not always better. I think hitting the body with so much fat and sugar at one time is not so good.

- I think most people can eat more by having variety, and eating throughout the day, than by stufffing one thing once or twice a day. So the shake will be lots of calories, but she might manage more total calories with a variety of foods all day.

- There are diifferent ways our body uses to tell us thatwe are full, besides how physically full our stomach is. Apparently things that are basically sugar water, like apple juice, get missed by most of them. So if she is really determined to gain quickly, clear fruit juice could help, for when she is not full of shake.

Me, for one.

Some people cannot handle a lot of fat at once, could that be it?

In an emergency, a seamstress should be able to add a small panel (or more often two, for symmetry). Takes some time and money, buts beats not being able to fit in the dres.

If you understand her fantasies better, might be able to do some play with them, without really gaining. Like having an evening where you pad your clothes to look fatter, and play the part of her fat pig. Or occasionally going to a buffet and really overdoing it ( and talking it up), or letting her hand feed you.

Just something to think about. Think of it as (fat) porn for her, turning her crank via fantasy.

Taking a month, or longer, and drive across the continent with a mutually gaining partner. At each stop along the way stuffing ourselves (buffets, weight gain shakes, local specialties, whatever), and constantly pigging out as we drive. Bellies constantly swollen to max, but capacity steadily growing.

We would gain a ton of weight, but not in any one place long enough for people realize we were gaining, or even how much we were eating overall.

At the other end of the trip people would first get to know us as already really fat. And we might think our stuffing days were done, but it would be hard to stop after that long.....

What you describe is not that uncommon here, I'd say.

"Massage" is maybe what you meant?

Yep, that is why I figure it is best to stop gaining/really limit your gaining at as low a weight as you can manage. If you are going to be wanting to gain but resisting, might as well be as small as you can otherwise bear--makes the rest of life easier.