Parece que você definiu o dispositivo para "centralizar" as imagens e manter a proporção. Você provavelmente quer olhar para configurá-lo para "esticar / zoom" para preencher a tela.
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As opções são:
KEEP Aspect ratio will be kept, and the image will be scaled down or
up so that it fits inside the box, with transparent bars either side
if the ratios are not the same.
STRETCH Aspect ratio is not kept - the image will be stretched so
that it exactly fits the box.
SCALE Aspect ratio is kept, and the image is scaled up so that it
fills the box, with any portions that lie outside the box being
cropped off.
CENTER Aspect ratio is kept, and the image is left unscaled, centered
within the box. It may be cropped if the image is larger than the box,
or may have a transparent border if the image is smaller than the box.