To see the difference that these fitting types make, turn on the display of the Control Points. These points can be viewed by checking the Control Points checkbox under the Display tab in the Geometry Display > More section.
Constant Fitting - Constant number of control points between patches. M by M Matrices are built, where m is the Control Points setting in the Fit Surfaces dialog.
Adaptive Fitting - Notice the larger and smaller matrices of control points between the patches. This uses an M by N control point layout rather than M by M .
Adaptive fitting will typically produce a much smaller file size because it optimizes the number of control points within each patch.
See the related knowledge base article: Are there any tips to reduce the file size of a part I have created in Exact Surfacing?
This method also allows the setting of a fitting tolerance to the original polygonal mesh.