If you struggle to eat enough, the answer is healthy fats, not bigger portions and not smoothies.
Fat has more than double the calories per gram of protein or carbs, so it adds calories without adding volume.
Easy ways to add them:
A lot of butter when cooking your eggs
Honey in full fat Greek yogurt
Sourdough with butter and honey
Butter or olive oil on rice and potatoes
Staples to keep around: nuts, olive oil, full fat milk, full fat Greek yogurt, cheese, red meat, salmon, eggs, oats, honey, dried fruit
Four or five smaller meals beat two big ones if you don't like large portions.
Make sure you're on the Muscle Builder meal plan (a slight surplus), and match your target at the top of the Nutrition tab with bigger serving sizes.
Keep the strength work in. That's what turns the extra food into muscle instead of fat.