The common vocal issue of a too heavy falsetto voice is connected to the chest voice (or closer muscle) not working well, or maybe not working at all.
When the closer muscle does not do its job, then the other main muscle, our stretcher muscle, has to compensate. When it does this, then it becomes overworked; trying to do the job of closing that it is not designed to do. The muscles have to do their own specific jobs and be in right relationship with one another. When this does not occur, the voice becomes unbalanced.
The cords are supposed to be thicker and longer on the lower notes, thinner and shorter on the higher notes. For us, we would have a sensation of heavier on the bottom and lighter on the top. However, when the stretcher muscle is doing all the work, then often what we feel is that the top has gotten heavy and the bottom of the range has become light and seemingly weak.
The stretcher muscle basically becomes a disgruntled tyrant doing too much, in the wrong way. After awhile the voice disappears as overproduction takes its toll.