Long story, short: I sent some Ficus sycamorus cuttings to a fellow at Gitmo, who wanted to root them. His response (after success) was:  "The small stem sycamorus set much vegetative growth and exhaust themselves before rooting. The large stems are slower to flush out and have time to root."

The thinner stems would have been from the tip of the branches, where vegetative growth would have been more recent, and the thicker piece from further down the branch.

I didn't know if the wood root, and there is not such thing as dormant cuttings. So I sent non-dormant cuttings of various thicknesses with the leaves stripped off and about 12" long. I don't know if this relates at all to rooting Ficus carica cuttings, but if you are trying Ficus sycamorus, this may help.

Last summer; to make one for a fig-friend,  I cut three ~12x1/8"
green tips off my F.s.. I cut off the very bottom leaves,
put in a jar with 2" water and covered with a loose plastic bag.
Waalaa, they rooted like crazy (in basement next to a window).

Good thing I did that; my little mother-plant is now dead!
F.s. is a very, very tender specimen - more so than the annual
Impatients plants (40*F+)! I thought that I had brought it in just in-time,
but is was still too late.
Sooo, what I have now is only the other 2 "spares" and are still potted in foam cups....

Mature F.s. trees are awesome! I am growing it only just for
curiosity and do not expect it to mature or bear any fruit here
in cold NJ.

Thanks for the info and thanks to Ingevald even if he has nothing to do with this post!

What an interesting thing to learn!  I have a F. sycomorus in a 10" pot, it's about 2.5 ft. high with a few slender branches. I have it sitting just inside my patio door right now. 

I don't expect mine to ever get very big or to bear, either. The main reason I got mine was for historical significance and of course, because it's a fig!

To see how awesome a mature Ficus sycomorus fig tree
looks like in one of its natural environments;
see Ray Givan site:

http://www.raysfiginfo.com/sycomore.html

Also, it has biblical values.
Yeah gorgi, keep dreaming on...

I have two of the same age, one in a 15 g pot, and one in-ground. The one in the pot is about 3' wide and 7' tall. The in-ground one is easily 15 feet tall and 12 diameter in the same time. They apparently require a lot of root space to really thrive.

Gorgi, the main reason I got my F. sycomorus was to add to a garden with a theme of plants mentioned specifically in the bible.