Age. If you grow it in the right conditions (light, water, etc.) for that plant, then it's just a matter of time and you can't rush it. Certainly do not leave it in a tiny bonsai pot - that can come much later (years) when it's the size you want to keep it at.

Thanks for the effort, but that didn't really answer my question about how to get the base bulbous. I realize it takes a long time to accomplish this. I was hoping someone would supply me with a technique to accomplish this.

The tree will do it all by itself - it's it's natural growth habit (if you are growing a certain type of ficus - not all of them do that, and you can't 'make' it happen very well, it's genetic). This is not the same thing as, e.g. a pine or juniper which, if you use certain techniques on them, will develop fatter trunks (more quickly than just age alone will). Ficus are different and I doubt if you could achieve the same kind of results, but could damage the tree.

I have heard of a technique whereby you put wire just at the base of the tree at ground level and tighten it. I have never tried it and don't know if it will work on a ficus or which variety of ficus you are referring to. You have to be careful not to tighten the wire too tight or loose, and as usual with wiring, don't leave it on too long (don't know how long it would be to take effect).

You may or may not get what you are looking for, but if you have a lot of cuttings and want to experiment...why not. I would let the trees grow larger than just rooted cuttings before trying this as it may be a little harsh.

I say again...I have never tried it and don't know what your results will be. This may be one of the techniques Lucy was talking about and not mentioned because she has more experience and knows it will not work.

Best of luck

That's one of the things I referred to as being used in evergreens, but I wouldn't really want to try it on a ficus without further assurance from someone like Jerry Meislik (ficus expert extraordinaire)... if you want to google his website and ask him.