'Jettison some weight or I'll never make it to Cybertron'

Astrotrain would need to burn fuel for deceleration and course corrections later on as he approached Cybertron. Perhaps he made his internal calculations and determined that he had already burned too much with his load of passengers.

And because Astrotrain had to collect all of the surviving Decepticons and take off from Earth in a hurry at the end of the Battle for Autobot City, it is unlikely that he was as full on fuel as if he were just sitting around preparing for a long space trip. So he probably started out with less than a full fuel tank to begin with.

Then consider that he made his getaway from Earth with the surviving Autobot and Earth defense forces trying to shoot him down, so he was probably accelerating from Earth on absolute maximum power. This means that he was burning through his fuel supply at a very high rate.

Since he had just been through the battle along with all the other Decepticons, it is possible that some of his systems may have been damaged. He might've gotten his navigational array dinged, or had one of his reaction control thrusters taken out, or gotten shot in his fuel tank which caused a slow leak. Some of these damages could've resulted in him taking less than the shortest, most direct route back to Cybertron, and that means an even higher than expected fuel burn rate. A fuel leak should speak for itself.

Hell, maybe he just had a bad batch of Energon which partially clogged parts of his fuel pumps, fuel rails, O2 sensors, fuel injectors, and valves, which resulted in lower-than-normal fuel economy. Anything's possible!

I know what you mean. Even though the scene defied so much rationality, it was still one of my fondest memories seeing the Constructicons have a brawl with Soundwave and his tape crew; where the Constructicons all combined into Devastator to meet Rumble and Frenzy's bluster, with both Rumble and Frenzy transforming their arms into pile drivers to pound on the ground and shake apart Devastator back into the Constructicons. Good times!