The figure doesn't look too thin to me at all. The figure looks skinny compared to the movie stills because they're not displayed in the correct aspect ratio, making Jason look shorter and fatter than he really is in the movie.
Richard Brooker was slim but very fit when making this movie. Part of the Jason costume that he wore was padding to "bulk" him up to what the Director wanted. This was very apparent as you watched the film. You can make out the padding going over his shoulders and all around his body. That look is not recaptured with this figure. That padding and bulk also help making the shirt he was wearing undersized. As a stand alone figure, it looks nice. But as a figure that is meant to represent what was seen on film, there is lots of inaccuracies. What makes me confused about this is that this movie came out over 30 years ago with many behind the scenes references available on the internet as well as the films themselves. I'm not sure how, at this point in time, can you not make the perfect Jason 3 figure. I've been told there is a delay with this figure. I hope it's for improvements.