Something that is ponderous (1) has great weight, (2) is unwieldy, or (3) labored and dull. Ponderous does not mean thoughtful or given to pondering. Ponderous and ponder share an Old French root, but they came to English separately and have always had unrelated meanings.
Ponderous is almost always used figuratively and is not simply a synonym of heavy. It wouldn’t make sense to describe a 100-pound package, for example, as ponderous just because it’s heavy. But getting the package into your apartment might be a ponderous task.
If you need an adjective meaning characterized by or given to pondering, consider thoughtful, pensive, contemplative, and wistful, and there are many other synonyms.
Examples
It’s natural to assume that ponderous and the verb ponder are related, so confusion over the meaning of ponderous often occurs. For example, these writers use ponderous as a synonym of thoughtful or deep-thinking:
But the continuing shriveling of banks’ market value means ponderous bank analysts have been wrong about the “They’re Cheap!” call so far. [Wall Street Journal]
In his ponderous vocal style, Bazan revisits some themes from early Pedro the Lion albums. [Christianity Today]
And these writers use ponderous in its traditional sense.
Compared with NBC’s ponderous, effortful performance this morning, Fox’s presentation felt relatively light. [New York Magazine]
He stayed at the same hotel to facilitate quicker absorption of the Ravens’ ponderous playbook and help the rookies get into the flow. [Herald Mail]
Readers new to the field may find this book a useful overview, but only if they can endure the ponderous writing. [The Economist]