If it's the little ones snack bag and they're bringing so much to your house I'd ask if they could let me know how much and when the child usually eats and if they are expecting all the food to be eaten whilst in my care? Maybe dad just feeds the child a large amount and doesn't understand why the bag always has food left in it and is trying this tactic to show you that he wants the child to eat more whilst at yours and is too soft to say anything to you too? So you're both sort of playing the same game - you think it's too much and haven't given it to the child and maybe haven't mentioned it and presume by giving the bag back with food in it implies enough of a message that it's too much? You may well have mentioned it before - I am just playing devils advocate.

It definitely needs one of you to approach the subject and maybe dad will be pleased to understand your point of view as much as you will enjoy him understanding it. Well worth a chat.