To clarify, when using the "Show Size in Tons" button, the displayed size changes. A ship that displays as 10,000 Tons after making can become 10,005 Tons by simply pressing the "Show Size in Tons" button. I believe it might be a rounding error or display error.
I add several components, I get 10,000 Tons. I click the button, now it is 10,005 Tons.
If I go long enough without clicking that button a few times, I can go from 10,165 or something down to like 10,030 Tons.
That's the "bug" part.