The smaller cavity may not provide the required clearance around the dish. A physically close fit can obstruct heat paths or interfere with the door.
What the observation means
The top oven's rated capacity and accessory dimensions matter independently of the main oven. Handles and lids can extend beyond the dish's base dimensions.
A useful next step
Use the model's recommended cookware and shelf arrangement. If the meal is too large, choose a suitable approved cavity rather than forcing the dish or leaving the door ajar. Report any contact marks or changed door closure before further use, instead of trimming accessories to fit.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.