Usually burgers can be very high calorie if you design them to be.
My high (healthy) calorie burgers usually include black beans(or kidney since for black ones I need to travel to a certain store to get them here in Cyprus), some grain (be it quinoa, oats, bulgur wheat or smth similar), nuts (usually cashews[roasted but unsalted] but you can try something else), rarely I add whole or broken seeds but I usually add flax seed meal as a binder plus some vital wheat gluten. Beware though, adding too much VWG and adding wheat/rye/spelt flour also, will make your burger taste too "bready".
So, this burger is the direct opposite of a meat burger cause everything in it doesn't contain much water and most ingredients (beans, quinoa, nuts, seeds, vital wheat gluten) are high on calories when related to their weight.
Also a vegan dip is always a good idea. Try avocado + artichoke hearts +cashew + maybe some nutritional yeast + lemon juice and herbs to taste.
Also oatmeal as a breakfast with seeds, soymilk (or other) plus some fruit can give you a hefty amount of calories.
I also love some falafel with some veggies and hummus in a pita.
General rule: Legumes, beans, seeds and nuts are the best bulking foods for everyone since beans and legumes by dry weight are 20-30% protein 40-50% carbs and some very low to no fat (except soy) and nuts/seeds are 23-26% protein 50% fat and 10-12% carbs if I remember correctly. So the rule a grain, a green and a bean really applies here (plus seeds and nuts though).