For general heaviness:
* ponderous: This word suggests a weight that is physically or mentally burdensome.
* massive: This word emphasizes the sheer size and weight of something.
* bulky: This word refers to something that is large and difficult to move.
* weighty: This word can refer to physical weight, but it also suggests importance or seriousness.
For specific contexts:
* arduous: This word implies a task that is difficult and tiring.
* burdensome: This word suggests a weight that is oppressive or difficult to bear.
* grave: This word refers to something that is serious or significant.
* oppressive: This word implies a weight that is crushing or suffocating.
The best word to use will depend on the context of your writing.