By default (No) the meal list on the subscription meal-selection page only offers each meal in the plan's portion size. With Yes, the meal pop-up on that page also lists the meal's other portion sizes, and picking a larger portion adds an upgrade charge on top of the plan price (based on the price difference configured for that portion). If the subscription uses per-category portion sizes (the Meal Plan: Portion Size per Category / custom-portion-plan feature), the upgrade option is suppressed there since the portions for that plan are already fixed per category.