Usually, the grooms cake is smaller and may or may not be counted as part of the total servings. If it is a unique flavor, some guests will want two pieces of cake, sometimes not. I have found if you offer multiple flavors of cake, you will need more servings that just for the number of guests. Many of them want to taste all the cake.
It depends on the groom, really. Years ago, when I got married, I had a cake that fed 200, and the groom's cake was a 9 x 13! And we had plenty of cake.
Some grooms want theirs as big as the brides, some just want a small one. You have to go by what they want.
Generally, if the groom's cake is chocolate, and the bride's cake is anything else, they may want the groom's cake bigger than they would otherwise.
I usually tell my clients that the grooms cake is meant to feed about 40%-50% of their guests. It works out pretty well.
I want to say that it should be around 30% of the servings needed. Of course I don't think there is any steadfast rule, so if you need to make more servings to complete what they want then that would work too.
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