This must be a bug, surely. All these superlarge stars are 300 solar masses exactly. From memory, stars heavier than 80 solar masses aren't thought to ever get cooler than yellow at any point during their evolution.
Seems probable. The largest star found in 0.9.7.3 (the version I'm still running), is to the extent of my knowledge still 15.993 AU. I believe the largest star known "IRL" is also around this size. In the next version (0.974) there are suddenly stars more than 10 times this diameter ?