You should give her the big cake, if that's what was discussed and promised at the consult. If just a three tier cake was discussed, I think you're fine to make the smaller one.
And if it's your friend, why does your DH care if you look cheap? Not that I think you will. I've always not given a birthday or wedding gift if I've done the cake, and in every single case, people have told me it's too big a gift, not too small.