How can I make a bar cookie more dense?
The usual answer is "cook it less"

Certainly a (IMHO) good fudgy brownie and a (IMHO) horrid cakelike brownie can be made from the same recipe, with only the cooking time changing. While I have not made your mincemeat-based-bars, the same phenomenon presumably applies.

If you are faithfully following the cooking time your mother used, it's possible that the oven your mother used and oven you are using vary wildly in actual temperature at the same set temperature.