Look a little closer
A broad silhouette does not necessarily mean a large usable interior. Seams, compartments, a narrow opening and a curved base may affect what fits. Start with a familiar bag and describe its outline separately from the things you can comfortably place inside it.
A note for your design book
Use a few ordinary, non-fragile items for a personal fit check and note the opening as well as the interior. Do not generalise the result to a product you have not handled. A design sketch can then show where space is needed. Keep the result as a practical observation about that bag, not a standard capacity certification.