system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are rocket exhaust ports increasing in diameter, while jet engine exhaust ports are decreasing?
gpt
Wasn't going to answer, but a lot of these answers are missing the main point.

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The point of an an engine is to shoot gas out the back as fast as possible. The faster the gas goes, the more thrust per mass you get.

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For gas flows below the speed of sound, increasing the pressure makes them faster. All nozzles, jet or rocket, will include a section that decreases in size. This accelerates the gas until it reaches the speed of sound, and for most jet engines that's all that we do.

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However, rocket engines really need more thrust per mass of gas expelled, because every gram of gas expelled takes up space in a very limited fuel tank, so they accelerate the gasses even more, using what is called a de Laval nozzle (converging-diverging).

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For supersonic flows, the effect of pressure changes. Now, low pressures make the gas go faster, so we expand the gas as much as possible as soon as our flow reaches the "throat", where the gas flow reaches the speed of sound.

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Of course, we can't expand the gas until it has lower pressure than the atmosphere, because then the atmosphere kind of pushes back into the nozzle, and we can get some really weird shockwave-type effects. But that's talk for another day.