I'm not an expert, but I do know a tad about the Yang family history.
YCF did standardize the form we use today but I don't know that you could claim he "invented" it. I've always seen the word "standardized" used when people are discussing how YCF modified the large frame form.
The large frame form clearly existed prior to his standarization of it. The way I understand it YCF evened out the pace, prior to his standardization the form was done at varying speeds, both fast and slow at different times, he made the pace all one even tempo. He also removed the leaping and stamping movements from the form, practices that can cause physical damage to a practicioner if they are not done correctly. I have heard it said that he also removed the outward expression of fa-jing from the form, opting instead for a smoother, more rounded and continuous set of movements.
All of this information is fairly easily available on the web by Googling Yang Cheng Fu and reading the pertinent articles.
History clearly tells us that the Yang large frame form is what the Yang family taught to the general public for two generations prior to YCF's existence as a teacher. It was the frame that Yang Ban Hou first taught to Wu Chuan Yu before Yang Lu Chan then taught him the small frame, so we know it existed as far back as Yang Ban Hou, at least. Since Yang Lu Chan took over classes for Yang Ban Hou, which is where he met WCY and taught him the small frame, one would have to imagine he was well versed in the Large Frame in order to do so. From that alone I would say we can trace the Yang family large frame form right back to the founder of the Yang style.
I have knowledge of and at one time actually practiced a version of a large frarmed form that is verifiably tracable back to Wu Chuan Yu, and it was very similar to the Yang Cheng Fu form. The only differences are in the timing, both fast and slow movements are practiced. All the leaps, the stamping movements and fa-jing expression are intact.
Other than those changes, there is very little difference between them however.
Just my observations and research on the subject. As I said, I'm not an expert on Yang family history just a guy with some small amount of knowledge on the subject hoping to clarify the issue.
Others will have other opinions, and that is fine as well. It's not the style that matters, or its history, it's the art.