Card thickness.
yes.
Different thickness's.
I have an old USG and a later USG greenie (both what would be called white box C). The difference in the thickness of the cards is very noticeable.
And put the two decks side by side and and you can see how different they are.
Apart from that the cards are the same size (height/width) and that also is the same with the older Llewellyn/Weiser editions. Same height/width.
The white edges round the backs on modern decks make them look smaller than the older decks where the backs went right up to the edge.
There is really no difference in the card size, but the back image is, of course, smaller and it can be deceptive.
Also, on some of the very old decks in the 2 part boxes some cards are faded (printer error rather than age) or double printed.
If hers was like this then a modern deck could indeed have looked 'changed' to her.