I don't have a solution for you, but the reason is the the save as an image does a screen shot of the current graphics window - you can see this by zooming in or out before making the jpeg.
Welcome to the forum,
I would suggest if you have access to it using a program like Snag-it or one like it
then can do a screen capture and save it as a .jpg without all the distortion.
What if you printed it A4?
Before printing the document, I adjust scale to 100% from the page setup menu, and there is no problem with the print out. The rectangle coincides with the edges of the paper.
Instead of saving your model, how about you create a drawing from this "model" and save that to a PDF file.
Make sure you set your margins to 0 and your scale to 100% and it should fit perfectly (except for those dimensions, they should "fall off").