John Carter is considered to be the greatest swordsman on Barsoom (Mars). This is in part due to his extensive skill (even he doesn't know how long he's been practicing arms) but in another part due to his relatively high strength. His muscles are used to Earth gravity, so while on Mars he has a distinct advantage. In my short story Just Across Town, the protagonists' off-world physique both helped and hindered them – they were stronger than the locals, but unused to the lower air pressure and oxygen levels.
The Gravitational Effects Table (ST Charts book, p. 5) says that you can carry or lift more when on a lower-gravity planet. In the table, the difference is expressed as a percentage of standard carrying capacity. The standard load is 1kg of weight for each STR point. So a person of 'typical' strength can carry 7 kg worth of gear before being 'encumbered'. Lower gravity means you can carry more without penalty.
It seems reasonable then, that such changes can also be expressed as an effective change in STR score. Multiply the base STR by the load percentage and round up. This becomes the effective STR while on the low-gravity planet. This in turn will have an effect on a character's ability to fight with melee weapons.
For example, Bruce, an NPC character has STR 6 and Blade-1. He avoids the -DM for STR, so his total DM is +1 (from his skill). When Bruce visits a size 4 world (150% load) his effective strength is [6 x 1.50=9] 9. Now, the Advantageous STR score for Blade is 9, so as long as Bruce remains on this lower-gravity world, his effective DM when fighting with a Blade is +2.
If Bruce visits a heavy-gravity world of size A (75% load) his effective strength is [6 x 0.75=4.5] 5, which may mean he incurs the -2 DM for required strength.
The gravity effect should not reflect a character's ability to absorb injury, however. This only effects muscle power's interaction with local gravity.
This requires little math to calculate, while adding a layer of reality that can influence player decisions. Mercenaries or explorers might be reluctant to take jobs on heavy-gravity worlds because of the drag on their abilities. Variables like gravity and air pressure give worlds their character and flavor, and keep them distinct.