That certainly seems counter-intuitive. More “fuel” equals shorter life???? Hmmmmm.
More fuel = burns much, much, much faster.
Although it is fission rather than fusion you can compare it to having some U-235. A little bit will just toss out some radiation and last for millions of years. Add more and it gets more radioactive as spontanious splitting of the atoms cause others to split which will use up the uranium faster. Add some more and it only lasts for a fraction of a second before it blows up.
More fuel = more mass = bigger reaction = higher outward pressure countering gravitational force keeping the whole thing in a nice, neat ball and the furnace going. When that pressure overcomes the gravity, the outer shell is blown away, or the whole magillah starts expanding until equilibrium is reached again. Blast furnace needs more fuel than a campfire to keep going for the same amount of time.
Yep, though they have a lot more hydrogen fuel to burn, they go through all of it much faster than a smaller star goes through its supply. Think top fuel dragster. Fifteen gallons of gas is enough for a small sedan to go 300 miles. But a top fuel dragster almost burns that much in a quarter mile.
It does seem "counter-intuitive", but the rate, at which the fuel "burns", goes up exponentially with increased stellar mass.
There are a number of factors to take into account.
First, the more fuel in a star, the more massive it is. Both the temperature and size of a star are proportional to the mass. As it turns out, the brightness of a star (or rate of energy escape) is proportional to both the temperature (to the fourth power) and the diameter (squared), for a double whammy.
Second, the hotter stars undergo a different type of fusion (carbon-nitrogen-oxygen cycle) than the sun, and that type burns a little faster at higher temperatures.
There are also other factors like mean molecular weights and convective mixing that change the amount of fuel in a star that may be burned, again favoring a longer life for smaller stars.