In the Baeyer-Villiger reaction primary alkyl groups are much less prone to migration than secondary alkyl or aryl groups. Generally the more substituted group migrates. The transition state of the reaction places a positive charge over the molecule, meaning that the more stable that positive charge the faster the rearrangement. However this is not all of the picture. Some groups are SO cation-destabilizing that they migrate so that they are not left next to the cation formed by the reaction.
Stereochemical factors are also important.
When R is Methyl, around 90% of reaction goes by migration the Ph group. If R is tertiary-butyl, the figure is around 2%, with about 80% proceeding via migration of the R group.