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Marc S.you need to look at each edge under a loupe. I was a very advanced collector of the 1955 Bowman baseball set for many years...and that is one of the post-war sets that has the highest variation in card size, from the factory. Within a single pack, cards could be 1/8 to 1/4" oversize, or undersize. So from a card alterer's perspective, it is much easier to take an issue like that, trim down an oversized example, and have a card that "measures correctly", but is still trimmed.
The T-206 set is another example of this. I have heard many say how the average size of a T-206 has gotten increasingly smaller over the years.
You can also look at the opposite side of the argument. Just because a card is too small, it does not imply that it was trimmed. It only implies that it is small -- whether by trimming or because it was factory cut short is a different analysis altogether.