Let me preface this by saying that this is outside of my area (so to speak), but as far as I know...
Then you're only measuring the height and width of the card. In some cases they're pressed and trimmed so the area ends up back where it started, but the total volume and mass of the card have decreased. That's harder to detect. It's not that there aren't ways that such slight changes can be detected (If you want to get fancy you could put the card in a chamber with a known amount of an inert gas and then measure the pressure inside the chamber to calculate the volume of the card) but that the normal variance in sizes of vintage cards is large relative to the amount of matter discarded in a slight trim/press, so even if you knew the mass of the card to the microgram it wouldn't be definitive. Some cards might be suspiciously flat, but even the thickness of the cards will have some variance that's just due to differences in humidity, storage practices, etc. over the years.
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