Remember a decent wedding breakfast (stupid name for dinner!!) will cost at least �60 per head often much, much more. So if money is an issue they will lighten the numbers for the actual day.
However, I personally see this as a negative thing. If I was only invited to the evening do, which I have been, I only spend 20 odd pounds on a gift, or nothing at all. Yet if I am invited all day, I am certainly more generous and spend way above the cost of my dinner.
Bear that in mind Rubes. No dinner means smaller gift!!