Not just large white-headed gulls, but birds in general, will appear to have varying head structure (and body) depending on behavior and "emotions". That's right, emotions. A bird feeding will not look the same as a bird in a relaxed state. A bird alarmed will not look the same as a bird that's preening. Environmental factors also play a role in this subject too: wet feathers, versus fluffed feathers on a windy day can substantially change the bird's appearance.