I haven't studied xiaojia, but that is the form that Chen Xin did and his book is the Chen taiji bible. There are a couple minor differences in the form, but the body requirements all seem the same as what I have been taught.
If the same body mechanics as the same as large frame but smaller, it’s wrong. In Xiaojia, or small frame, the alignment of structure is strict, detailed and the connections direct. Not dantian moves first, sequentially out to the fingertips. Instead, everything moves together at once. Thus the expression of xiaojia is more compact, looks more simple as the mechanics not as evident.
“Once Qi of the hand moves to the back of the foot, then big toe simultaneously closes with the hand and only at this moment (one can) step firmly.”
“Whether foot is Empty (Xu) or Solid (Shi) depends on hand, if hand is Empty then foot is also Empty, if hand is Solid then foot is solid too.”
The teachers who focus on Xiaojia regard it as harder to understand and a progress from the evident to the hidden and invisible. They also regard it as more martial and more practical. The large frame should be taught first, but not all Chen stylists progress from the large to the small.
Or as Chen Xin’s most famous student Chen Ziming wrote:
“I have heard that my late father’s Taiji skill was at the peak of perfection due to his “vanishing circles”. This is not something that is easy to come by. You must proceed step by step, shrinking from a large circle to a small circle, then shrinking from a small circle to a vanishing circle. You will then be using internal power to control an opponent, transforming constantly, wielding with ingenuity. When skill is at this level, one’s postures are totally unreadable.”
This is why I find that it’s strange that all Chen stylists seems to be overly obsessed by keeping the external look of the large frame what ever they do. They seem to believe that everything should be externally explicit and very few aim for what is meant in that golden old saying that once described the old Neijiaquan: “Power is generated from emptiness.”