When it comes to many upgrades, not every upgrade increases capacity. Some oven, grill, stove and fryer upgrades increase only the speed with which a food item is prepared.

This is prominently displayed just before tapping the 'buy' button in measurement gauges; green being the upgrade level of what you currently have, the yellow showing how much an upgrade's attribute will be increased upon purchase.

Keep a sharp eye out for exactly what you're getting when purchasing upgrades, whether it focuses on just speeding the appliance up or increasing its capacity. Sometimes it's both! 

You can also double-check anytime in your upgrades by how many circles are filled-in under the Appliance, Decor or Food item's name in the upgrade list.