I feel those images are a little misleading. One might erroneously assume there is a dense surface akin to our star at the outer edge of these massive stars but..
"In the outer reaches of the photosphere the density is extremely low, yet the total mass of the star is believed to be no more than 20 M☉. Consequently, the average density is less than twelve parts per billion (1.119 × 10−8) that of the Sun. Such star matter is so tenuous that Betelgeuse has often been called a "red-hot vacuum"."
Also: "estimates of its mass are poorly constrained, but range from 5 to 30 times that of the Sun". Amazing for having on the order of 16 billion times its volume.
How does that happen? Is its fuel more energetic in some sense or does the gravity-vs-pressure equilibrium simply favor lower densities for a total mass of 20M☉?
You're right on both counts! Betelgeuse is burning helium now (post main-sequence). The higher output of helium fusion inflates the outer atmosphere like a giant floppy balloon.
"In the outer reaches of the photosphere the density is extremely low, yet the total mass of the star is believed to be no more than 20 M☉. Consequently, the average density is less than twelve parts per billion (1.119 × 10−8) that of the Sun. Such star matter is so tenuous that Betelgeuse has often been called a "red-hot vacuum"."
https://en.wikipedia.org/wiki/Betelgeuse#Density