Matt’s information is one very common understanding of these terms. Since terminology sometimes implies certain viewpoints that are not universally accepted, I would like to expand a little on what Matt posted.
As I understand it, the contrast between New Frame (xin1 jia4) and Old Frame (lao3 jia4) is one that is applied to Chen Style, but is not generally applied to other styles. As is implied by the names, Old Frame came before New Frame.
From what I vaguely recall, New Frame originated with a somewhat recent generation of the Chen Family (within the last 150 years?). I do not recall much about the differences, other than that some practitioners have disapproved of the innovations, arguing that they violated principles of the older form. On the other hand, I believe that the New Frame is what Chen Fake practiced, and he was among the most famous Chen Stylists of the last 100 years or so. I also think that the New Frame has become the dominant form and de facto standard of Chen Style.
The term “big frame” is most commonly applied to the version of the form Yang Chengfu developed. There are reports that describe and contrast the characteristics of some of the earlier Yang family generations. There is no accepted documentation of Yang Luchan’s form, so one must rely on the widely differing oral traditions as to the details of his form or forms.
If I remember correctly, the reports describe the “Taijiquan” of Yang Jianhou, Yang Shaohou, and Yang Chengfu. I believe that the term “Taijiquan” can ambiguously apply to the entire art or to the principal barehand form. From the context, I understand the reports to be describing different characterics of the way in which these individuals did the form. It is not clear whether the form itself had significantly different details. The same reports talk about the fact that different members of the Yang family of those generations were noted for excellence in different martial skills.
Yang Jianhou was said to practice medium frame; Yang Shaohou, small frame; and Yang Chengfu, “big frame.” I do not recall much description of Yang Jianhou’s movements, but Yang Shaohou’s were described as relatively small and quick. Yang Chengfu’s form was big and expansive.
Different schools seem to view these characteristics differently. Some describe their curriculum as containing all three frames, which they practice for different effect or at different levels of skills. I think there is also some difference in the form movements themselves.
Other schools seem to view the frames as inconsequential variations of the same thing, allowing a practitioner to choose freely between them according to intent, level of skill, etc. Yet other schools have opted for small or big frame, exclusively, seeing the other frames as unnecessary to study within their particular transmission.