Leaf size on ficus has several factors:
Normal growth pattern is a small leaf emerges first, the next is a bit bigger and so on as the shoot extends until the leaves reach full size. Continually trimming back to the first leaf on all shoots will maintain small leaves.
All trees need a certain area of leaf (solar panel) to collect sunlight. That can be 1 huge leaf, several mid size leaves or 100 smaller ones. Encouraging ramification (mostly by pruning) will gradually see smaller and smaller leaves.
Both the above require pruning. Good bonsai maintenance requires judicious pruning. Need to become comfortable with using the cutters.
Thin trunk is natural for young stock. Thickening is generally directly related to how much growth is on the tree and how many leaves are feeding it. For quicker thickening, feed well, provide optimum growth conditions and allow it to grow freely. That's obviously the exact opposite of bonsai. Your tree will get large and unsightly but it will thicken quicker. We are still talking in years rather than weeks and months so to achieve a thick trunk you need to be prepared to stay for the long term. After trunk thickness is achieved the tree can then be chopped back to the correct size and then move on to the process of growing and refining branches and canopy. Say 5-10 years for the trunk and similar timeframe for branching.
The alternative is to be happy with a thinner trunk and start trimming sooner to develop branching and ramification. The tree won't be as impressive as a fat trunk but it will be your very own bonsai.