A better way to tenderize a larger piece of meat is to cook it slow. A marinade would not make a big difference on a large piece of meat because it only penetrates a 1/4 inch or so. I have cooked several roasts that have come out very tender and juicy by using a rub and cooking it at a low temperature in the oven.
To illustrate, take a pork roast such as a Boston Butt (no, a butt is a large cask used in the olden days to ship meat) slather it with yellow mustard and cover with a rub.
Rub:
1/4 cup sugar, 2 TBL paprika, 1 TBL freshly ground black pepper, 2 TBL onion salt, 2 TBL garlic salt, 1 TBL kosher or sea salt, 1 tsp dried parsley, 1 tsp ground ginger, and 1 tsp cinnamon. Mix well.
Let meat sit in refrigerator for 2 hours. Take out and let come to room temperature (about an hour) to save cooking time or put it in the oven. Place butt in shallow roasting pan, put in thermometer, and cook at 250 degrees until done. After 3 hours cooking, turn the pan around half-way to account for temperature variances in the oven. Also baste it with apple juice (or water).
A 4 pound roast will take about 6 hours (but well worth it). After it is done cooking, let it sit about 30 minutes, covered, before slicing.
DtC