I have only one speculation to offer so far: Mami actually wanted Madoka to wish for a cake. I get the feeling that your wishes can come out different than you expect, or big wishes can end up leading to unexpected and unpleasant consequences. Even if it doesn't follow the staple setup of wishes being granted too literally or twisted in some way, most significant wishes will have a major impact on things. For instance, if her wish gave her mother a major promotion, that could turn her mother's life upside down. This is probably also one part of why Homura doesn't want Madoka to become a Puella Magi.
Another possible reason would be "equal exchange". In other words, a big wish could result in major costs. This would be something that Mami would be in a position to know very well, since her wish was very large:
Either way, I expect if this is the case it will soon be very apparent when the two make their wishes. In the current situation, I doubt they'd be able to think of anything "small".