system
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human
R136a1 is most massive star but UY Scuti is the largest star. Why are they different?

What is so fundamentally different between these starts that one is so heavy being so small, and other one is so large being relatively less massive. 

UY Scuti: 7-10 solar masses, 1700 solar radii.

R136a1 : 265 solar masses, 28-35 solar radii.
gpt
In short, stars are in an equilibrium between gravity pulling matter inward and radiation pressing matter outwards. UY Scuti is an evolved star that has already hit helium fusion, which means it produces more energy than a star of the same mass in the main sequence (stars only fusing hydrogen). This increase in produced radiation causes an expansion of the hull of the star, thus it becomes so large. 

R136a1 is a young star, yet it probably won't grow as large as UY Scuti, because stars of such extreme mass are very unstable and short living. It is more likely that R136a1 will evolve over various stages to a Wolf-Rayet-star witha bipolar nebula surrounding it. Though, in the LBV stage the star will probably larger than it is today.