DFW said:
Average world size in MGT Trav is 5 (2d6–2). Grav on a size 5 world is 0.45G.
:?
My MgT isn't handy at the moment, is that correct? Previous versions have size 5 at 0.625Gs.
Still low, just not quite as low. On to your main point yes I did long ago wonder about this. For MTU I came to the conclusion that this was the preferred gravity (that mainworlds were chosen and settled by the Vilani) based on their homeworld. So their homeworld would have been size 5 and they would have evolved to it with the lighter gravity giving them:
Better health and longevity (less wear and tear on the body fighting gravity)
Lower Strength (just -1) not needing as much to carry their weight.
More height and thinner (originally my "space-elves" rather than the Darrians)
Higher Endurance (just +1) for greater lung capacity (from the taller frame and being adapted to the thinner atmosphere - average here is also 5 you'll note), able to tolerate Very Thin (Atm 3) atmospherewithout a respirator for brief periods but requiring a special filter mask for extended exposure to Dense (Atm 8 ) atmosphere.
...and of course most ships (and other AG areas) have internal gravity set at 0.625Gs, in the Vilani area of the Imperium.
Note that most of the (my) TU has been extensively re-settled (invaded) by Solomani (post Rule of Man) so ships (and other AG areas) as well as most PCs and NPCs are Terran base stock (1G and unmodified stats).
How much of the above applies to a generic Traveller 'verse will depend on your own choices. But taking the world gen as the baseline will (should) inform those choices. As noted it will mean most people live on lower gravity worlds with thin atmospheres.