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I can account for why the image in your book is smaller when delivered directly to the device from Amazon as compared with transferring via USB.
The USB copy will be in KF8 (azw3) format and will have the same HTML coding that you placed into the book. In this case the image will be displayed 200x300 pixels on a 600x800 pixel screen or 1/8 of the screen area.
The copy sent directly by Amazon will be in KFX format for newer Kindles, including the Kindle 7th gen with up-to-date firmware. This is the format used for delivery of most books these days and enables enhanced typesetting features such as hyphenation and page flip.
During conversion to KFX format Amazon applies several fixups to the book's original formatting intended to increase readability. Images coded with dimensions in pixels are converted to percent instead to promote consistency of display across devices with widely varying screen resolutions. The exact details are complex and I don't understand all of the rules, but in the case of your book the new size is width:23.4375% which happens to be 300 (your image height) divided by 1280 (typical HD screen height).
The result is that the image is displayed at 23.4375% of the screen width on all devices. For your Kindle 7 that will be 140x210 pixels or 1/16 of the screen area, so that the image appears about half the size of the KF8 version.
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