First of all I need to say that I don't know the answer but I am waiting for some knowledgeable people to respond.
However, when photographing 'normal' subjects where the image on the film is significantly smaller than real life, the smaller the format, the greater the depth of field.
Obviously at 1:1 the format is irrelevant as the image is always the same size as the object.
At greater than life size magnification though, I would be interested to find out if the rule reverses or stays the same. i.e. if you are filling the frame with the subject and it is larger than life size my gut instinct says that the larger format may now have the better depth of field.
..... but I could be wrong!