You could eat all the leafy greens you can shovel into your face and not get enough calories. You would need to eat 9 pounds (4 kg) of kale to get 2,000 Calories. It is virtually impossible to get enough calories from leafy vegetables alone. Leafy greens are great for their nutrients, but they don't have many calories. Eat starchy food to fill you up.

https://www.forksoverknives.com/why-starch-carbohydrates-should-be-at-the-center-of-your-plate/
On a whole-food, plant-based diet, 75-80 percent of your calories should come from carbohydrates (mostly starch-based foods and fruit). And you don’t have to keep track; this will happen naturally if your diet is based on fruits, vegetables, tubers, whole grains, and legumes.

Eating starch is a satiating experience important to enjoying the WFPB diet. These are the foods that people around the world have thrived on for generations:

• tubers like sweet potatoes
• starchy vegetables like corn and squash
• whole grains like brown rice, millet, quinoa, and buckwheat
• legumes like beans, lentils, chickpeas, and lima beans

Eat enough so you're never hungry.

For the same amount of calories, a WFPB meal is larger than an oily, meat or cheese-centered meal. Here are some pictures and charts:
https://www.google.com/search?q=food+Energy+Density&tbm=isch
WFPB eaters that can't stretch their stomach that much need to eat more often to get enough calories.

Weight Gain - For skinny vegans by Dr. Klaper
https://www.youtube.com/watch?v=bkBjVDG8_EU

Go From Underweight to Healthy Weight With These Plant-Based Tips
https://nutritionstudies.org/go-from-underweight-to-healthy-weight-with-these-plant-based-tips/

How Do I Gain Weight on the McDougall Diet? I’m Not Joking!
http://www.nealhendrickson.com/mcdougall/030700puhowdoIgainweight.htm