• pl lunch = en lunch = meal at noon
• pl obiad = en dinner = main meal of the day
• pl kolacja = en supper = meal in the evening
The English term “dinner” can be used for lunch/supper depending on cultural context, because people have different habits in the world for when to eat their main meal.
The combination “small dinner” is a bit contradictory, because the main meal of the day should be bigger than the other meals. However “mała kolacja” is fine. It's just not an equivalent translation.