I agree that gravity didn't change much since the earth formed as a giant, red hot, roiling, boiling sea of molten rock about 4.6 billion years old. Roughly the same mass, give or take a factor of two the same diameter, and always the same gravitational constant.
But ... why do you dismiss the "faster spinning" possibility?
For the reason given by another poster. To produce that reduction in weight called for by Ted's conjecture, The earth would have needed a day of two or three hours, and would have needed it 65 million years ago, roughly.
Any reduction in rotation rate sufficient to produce the current day length would have released enough kinetic energy to melt the crust. Only the tidal effects of the moon are slowing the earth, and not by enough to account for twenty hours per rotation.
There is also the minor problem that mega-dinos account for just a few species out of millions. Why does every other line of evidence point to unchanging gravity?