there are smaller tuna portions in plastic tubs - I've seen them at some stores but also at Minimus.biz. there are also 3 oz cans of chicken. Canned chicken can be easily dehydrated, unlike any other kind of chicken; spread some on a cookie sheet, set the oven at a low setting, prop the door open with a wooden spoon and dehydrate away. You can pack as much or as little as you want in your meals.

Someone told me they've seen half size pouches of tuna at a grocery store - still looking for them myself.

packit gourmet (google it) has shelf stable meats of several varieties - I'm waiting for chorizo to come back in stock. Legumes and lentils make for good trail eats - I just made a lentil soup to throw in the dehydrator.