Miss Maudie is recognizing Jem's movement from adolescence to adulthood with the cakes she has chosen to bake. The small cakes are for the children--they have their own, and won't need to share. Adults, however, expect to share in the communion of spirit and food. She cuts him a slice from the larger cake--the cake which is a coming of age moment for him. A slice of understanding and adult knowledge that come with growing older and wiser. Miss Maudie is paying Jem homage as a "man" in bloom.
In chapter 22 when Jem, Scout, and Dill arrive at Miss Maudie's house and there are only two small cakes, at first they believe that Miss Maudie has forgotten Dill. However, they "understand" when "she cut[s] from the big cake and g[ives] the slice to Jem."
Miss Maudie sees that Jem is old enough to grasp the meaning of what has happened at Tom Robinson's trial. She responds to this by serving him from a "grownup" cake and speaking about the trial with him in an adult manner. Jem is deeply discouraged about the outcome, and Miss Maudie explains that not everyone in Maycomb is corrupt--for example, it was "no accident" that Judge Taylor assigned Atticus to the case instead of the unexperienced public defender.
Miss Maudie feels that Jem has grown up throughout the course of this trial. She bakes the small cakes for Dill and Scout; however, Jem's maturity has earned him a slice of the big cake. Miss Maudie feels that Jem is closer to becoming a man.