Humans are stranded on a world with .75 Earth gravity. How would that affect their physical evolution? Specifically, would they be taller, thinner, and have less bone mass?
This is similar to the following closed question: How can lower gravity affect evolution?. There would definitely be less bone density, it's a known problem already. But taller and thinner? Not necessarily. In fact, probably not.
At issue is the wondrously wide variety of humanity today. We already have short and tall, thin and thick, athletic and not so much... and every combination of those things. The only real guarantee you'd have is that their leg, hip, and back muscles would be underdeveloped compared to their Earth counterparts.
Maybe, maybe, the average would shift to thinner (not taller, though). (See addendum below.) But that's the average, lost within that wondrous variety previously mentioned.
Addendum: Now that I've thought more about it, it's more likely that the people of your low-gravity world would be fatter than their Earth counterparts. Unless their behavior changes with the low gravity, they're arriving with the same predilections for sugar, carbohydrates, fats, and, well... BBQ... that humanity has had since time immemorial. With a lower gravity, they're more likely to become fatter. If they became thinner, it would be due to cultural changes that set them apart more from their Earth counterparts than what any evolutionary changes did ("What? You don't eat BBQ? What kind of alien invader are you? That's so un-Texan! I mean, un-American!... I mean... You're not from Earth, are you?"). Finally, remember that evolutionary changes take a honking long time.
It Could Possibly Effect Our Developmental Morphology
There is a difference between developmental morphology and evolution. There may be morphological differences between a human who develops in normal vs lower gravity without a single change to thier genetics. However, it is hard to predict exactly what these changes will be since there is no real experimental evidence to suggest what long term slightly reduced gravity will do. We know about all sorts of issues with long term zero G living for adults, but nothing about long term exposure to .75G, or what effect lower gravity has on developing children.
Since we don't know, then any conjecture is equally valid. It is possible your humans will land on this alien world, and the very next generation of humans will be noticeable taller and skinnier just because they developed under different environmental factors than thier parents did. It's also possible that colder temperatures, higher atmospheric oxygen, or the nutritional qualities of the local food sources could cause similar changes in the next generation having nothing to do with gravity at all.
Evolution Is Much Less Probable
The issue with pretty much every "Will X cause humans to evolve Y?" question is that modern humans experience very little natural selection the way that animals do. Evolution does not happen just because some new trait would be nice to have, it also requires than the existing trait be so unfit for genetic survival that people with it are noticably less likely to raise surviving offspring. Because we modern humans have medicine, advanced defensive technology, and don't just let each other die off for being a little bit slower or weaker, then your short humans and tall humans will both have the same basic chance to pass on thier genes.
Even if short people for whatever reason had a distinct physical disadvantage, humans have a way of overcoming physical disadvantages through specialization of labor. This may lead to increased genetic diversity in the future with more people of both exceptionally tall and short frames, but not a general push to evolve in either direction.
As to comments about sexual selection, there is actually a lot of reason to believe that this is not a major evolutionary factor in civilized humans either, because we are a generally monogamous species. The kinds of species where sexual selection overpowers selective fitness are generally Harem Polygamies. In a Harem Polygamous species where you put together 10 males and 10 females, the #1 male gets all 10 females. This emphasis on being #1 means that not being sexually ideal is not an option. But, in a monogamous or polyamorous species, The #1 male generally pairs with the #1 female, the #2 male with the #2 female, etc. In fact, the #1 male will generally refuse to mate with the #10 female, even if given the opportunity. This means that even the least desirable of the males (in this case, the shortest) still has a good chance of securing breeding rights. Normally this means that sexually unfit characteristics get paired with selectively unfit characteristics for removal from the gene pool, but going back to modern humans being less likely to ever be selected against, it means that sexually undesirable traits are much more likely to be passed on than in other species.
So, the most likely reason you would see for humans to Evolve to be taller is if your world introduces something that selectively kills or sterilizes your short people or if civilization on this world collapses for a really long time so that the alien predators or constant warfare get to have enough generations of killing off the shorter slower humans to impact the gene pool, or if major and extraordinarily long lasting cultural changes were to take root.