SSD or YOLO all have problems to detect small objects comparing with Regional based object detection. https://medium.com/@jonathan_hui/what-do-we-learn-from-single-shot-object-detectors-ssd-yolo-fpn-focal-loss-3888677c5f4d should have some explanation. The problem is the feature map they use has very low resolution and the small object features get too small to be detectable. YOLO3 applies FPN technique to address that problem. The link above will explain it.

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