I'm not up on my Kindle models any more--I have an Oasis and it's my perfect Kindle, I've forsaken all others. The basic now has a touch screen, right? I *think* it has to do with the type of touch screen on the basic; I think it's the same kind as on my Kindle Touch, which had infrared sensors. You could use, say, a pencil eraser to change the screen. The Paperwhite, the Voyage and the Oasis, on the other hand, use capacitive screens like tablets. You have to actually use a finger or something with a capacitive tip.
So the plastic bag touching the screen is acting like touches--and sometimes multi-touching at the same time, which is why you get the font size changes--it appears to be pinching or spreading on the screen.
EDIT: You can test this out of the bag. Trying using a pencil eraser or something else that's not very hard to change pages on the basic and on your Voyage and see how they each behave.
Betsy