Weird star sizes

[Malkevin]

Can any astronomers out there explain why some stars are huge for their mass?

Take Omicron Ursae Majoris, a G class star thats 16 times bigger than Sol but weighs less than one Sol.
Being over 9 billion years old, a couple of billion older than Sol, I thought it might be something do with it entering the red giant phase of its life.

But looking at a nearby star, GCRV 1568, another G class star thats 12 billion years ago, but this one is only as massy and big as Sol.

 

[Windscreen Smudge]

The GCRV star is the outlier. Omicron Ursae Majoris is a real star that was manually input to the engine, and GCRV is one that is procedurally generated.

Bug report it.

Edit: but to your underlying question, yes.

Omicron Ursae is the size that it is because of it's stage of life.

 

[Sapyx]

No, GCRV 1568 isn't procedurally generated, it's a "real world" star too; "GCRV" is the "General Catalogue of Stellar Radial Velocities", a list compiled in 1953 of stars which are moving at noticeable speed relative to ours, so they appear to slowly drift position in the sky from year to year. Most of the GCRV stars are fairly close to Sol, because a star had to be either very close or moving very fast to have detectable apparent motion and therefore qualify for inclusion in the GCRV catalogue.

While one or the other of your two examples might be glitchy, it should be pointed out that stars can have all kinds of things happen to them that alter their vital statistics away from where they "should" be. This usually involves some kind of interaction with other stars. For example, suppose a star that's nearly run out of hydrogen and might ordinarily be old enough to be approaching red giant stage has a close encounter with another star, maybe a red giant with a tenuous atmosphere; the smaller star sucks in hydrogen off of the red giant, which both adds mass and "rejuvenates" the smaller star.

 

[DWorth]

All about density, innit? Our sun will eventually become a red giant large enough to engulf the Earth, but it's mass won't increase, just its size (if anything, it'll have slightly less mass by this point).

 

[Shintai]

Unfortunately there is a huge chunk of "bad data". Everything from stars that shouldn't be able to exist in terms of type/lifespan to the range of planets with like an earth type planet closer than mercury to a bigger and hotter star than the sun.

I guess the best way is simply to bug report them one by one. Since I doubt Frontier will take a pass over it all.