I have a really cool Bonsai Fig I’d love to let grow big I’d take it out of it’s current container which has been tight for the better part of 3 years and put it into a similar medium but in a larger container. Currently the plant is very healthy and happy, about 1 cubic foot dimension - so it’s small.
I like the size of this one, I wonder how long that would take to grow mine about that size. It looks just like it but smaller now.
I’m not certain what your are asking here. Are you simply trying to preserve the proportions of your tree in a larger size? If so, the answer is “sort of”. When you re-pot to a larger container, your tree will gain both girth and mass, since it’s a ficus, expect a lot of rapid root development, and annoying trunk growth. You will be able to maintain the look as established branches will grow relatively proportionately to the trunk. Don’t forget to add a nice dose of osmocote or your fertilizer of choice to the mix; and keep in a shady spot for a week or so.
To get a trunk like that, the tree needs to be old. Repotting it in a bigger container will help the ficus grow, but it’ll also get all spindly and less bonsai-like. So, then you can prune aggressively and after some regrowth it will have a thicker trunk and proportinate small branches like it should. But to make it look like a proper bonsai and not a mutilated tree will take some skill.
Isn’t a “reverse-Bonsai” = “a tree”? 
I would have thought it was a bigger than normal tree. Or maybe something like this:
“To make a reverse-Bonsai broccoli you need extra strength fertilizer, growth hormones 34f5 and 23g, and a very large pot. Your broccoli might grow as big as 3 feet tall and live for more than a year.”
I’ve had the Bonsai for three years and hopefully will be teaching my children how to take care of it.
On a tangental line, I teach a course on Eco-psychology and one of the elements of the course is the transferring of living things cross genrationally. In Japan and China Bonsai, and Bonsai type plants can live within a family for generations. It teaches a group of people to learn to be rooted in their beliefs and values within a group or family.
I’m somewhat of a green thumb, but I’ve never done a reverse bonsai like I described in my OP. The best plan may be to simply do nothing. The picture in the OP looks like a very old tree, I’m hoping for that size when I pass mine on.
Why do you keep saying “reverse bonsai”?
What do you mean by that. . .just moving a tree in a container to a larger container?
If you want a big trunk, typically you want to keep a tree planted in the ground, or a very large container for several years.
When I think of Bonsai I think of small, delicate, manicured - when I say reverse bonsai I am basically just asking how to make a smaller bonsai much larger. I suppose I’ve got my answer. Bad choice of words I suppose.
I’m dissappointed. I was hoping that the OP wanted to sculpt a full sized tree into a nice pleasing bonzai shape. That would be sweet.
Anyone know where you can get 150/0 copper wire?
I think I understand the problem now.
Bonsai is simply the keeping of trees in pots. Just by restricting the available root space the tree naturally becomes stunted. Consequently the larger the pot, the larger the root ball and you wind up with a larger small tree. You’ll not be surprised that the Japanese already have a sizing classification though I can’t remember if they are official terms.
Tiny (Mame) 1-3”
Small (Shohin) 5-10”
Medium (Kifu) 10-16”
Medium/Large (Chu/Chuhin) 16-24”
Large (Dai/Daizan) 24-48”
Now I’m not sure a Ficus is going to last generations but if you wanted to thicken it you’d move it to a larger pot allow the root ball and truck to grow. Then you’d likely want to prune it back heavily over a number of years to get it into the shape you want. Eventually you’d move it out of its training pot into a more permanent display pot.
A good book on this would be this one. At least I liked it, though I have a poor tree survival rate. 
Ah Got ya. You might want to try a grafting project. Go and buy a bunch a small ficus and bind the trunks together tightly with electricians tape. Over bind that with some heavy gauge wire. After a year or so you’ll have a massive multi-trunk look.