Calculating the Diameter of Jupiter through Image
Your barlow is marked "2x" but I'll bet it isn't.

How is your camera mounted on the barlow? More importantly, what distance is between the camera's focal plane and the barlow?

An experiment to try (or at least to think about).  Get an eyepiece extension tube (or improvise something) and put it between the camera and barlow, increasing the spacing. You will have to refocus a bit and you will get a different (I guess bigger) image size, even though the barlow is still "2x". To get an exact "2x" barlow you might need a particular spacing, and eyepieces are normally closer to the eyepiece than cameras because of the extra bulk of the camera.  (I think this is the most likely cause of your discrepancy; your camera is currently mounted further back than an eyepiece would be so projection is making the image larger.)

A second experiment to try: without the barlow, photograph the moon and see how close you get to its expected size.  If it's close, then our assumptions about the barlow are in question.

Third thing to try: are you absolutely sure about the distance from Earth to Jupiter? (It's probably OK but I'm always suspicious about online calculators.)