Linq Except with custom IEqualityComparer

You could try something like:

var differences = list2.Where(l2 => 
    !list1.Any(l1 => l1.Name == l2.Name && l1.Size == l2.Size));

Or if you prefer:

var differences = list2.Where(l2 => 
    list1.All(l1 => l1.Name != l2.Name || l1.Size != l2.Size));