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I'm not an expert to that so maybe I'm wrong. When looking at the documentation

http://docs.ros.org/en/api/sensor_msgs/html/msg/Image.html

It actually mentions that

uint32 step           # Full row length in bytes
uint8[] data          # actual matrix data, size is (step * rows)

the size is step * rows no height x width. The reason for that is probably the fact that for each pixel you might not just have 1 byte but 3 bytes (e.g RBG).