I have heard trimming the large leaves after every few months helps the leaves to grow smaller over time. Is this true?
Yes it is true, sort of. Removing the leaves of a bonsai, or defoliation, will cause a new crop of smaller, fresher leaves to emerge, giving a temporary reduction in leaf size, but eventually the new leaves will enlarge and that temporary gain will be lost.
The real value of defoliation is in that it helps to promote a larger number of growing points. This is known as ramification. Think of it like this; if a tree has more growing points the energy available to any one is less than it would be with fewer. In other words more leaves that are smaller rather than fewer, larger ones.
Be cautious though, you must know your victim, err subject, well and it must be in good condition or you may cause more harm than good.