When I try to sneak in larger portions, she notices it and leaves some. I can't get her to eat more. She can't walk very far either. You didn't say how tall you wife is, but if she is over five feet tall, she is underweight. Her muscle mass is probably smaller because of her low weight and low activity level.
Inactivity can cause a poor appetite, which in turn effects a person's strength.
When trying to get a person to gain weight, some usual dietary advice may be thrown out the window unless she has some other health problems like high blood pressure or diabetes. If she is on a special diet, you will need to see a Registered Dietitian to help you combine all her dietary needs, including weight gain. Since she won't eat larger portions, I would suggest four approaches for weight gain. Try one or all of the following approaches to help your wife gain weight. First, increase the calorie concentration of the food amounts she does eat.