I'm a small eater too and hate feeling full and heavy, so I see what you mean.
The way to do it is to slowly increase the quantities. So for instance for a week or two, add an extra piece of fruit or a yoghurt here and then, and see how it goes. No need to go overboard and make you sick! Training for a marathon also increased my appetite, now I can handle more food ahah
Also remember to target food that are high in energy (nuts and healthy fats for instance) as you don't need to have much to significantly increase your calories intake. You might also try to limit a little the foods that are low in calories like vegs. So for instance, as your lunch and diner look filling, try to reduce vegetables a little.
I usually train in the evening at about 6.
So it goes like this:
3pm/4pm: snack (fruit, glass of milk, some nuts)
Hope that helps.
Again try to vary quantities of food/timing to see what works best for you.
I like leaving plenty of time between snack and training so I don't feel heavy or sick, especially if training is a run. I have the shake as soon as I'd done with training as I found out the earlier is the better.
Often I'm not very hungry for the meal but eat anyway as food is critical for recovery and makes a lot of difference in the way I feel the day after. From what you say, seems to be the same for you
Sometimes I don't finish the meal or have a smaller one. Sometimes I have an extra snack before bed.
It's good to have a meal plan so you know what you're doing, but we're not machines either. It's normal to eat a bit more or less than planned.