The term "more imponderous" is used to describe something that is even less weighty or heavy than its counterpart. The antonyms for this phrase would be "heavier," "weightier," "burdensome," or "ponderous." These antonyms suggest that the subject of discussion is substantial, serious, or difficult to move.
For instance, a feather compared to a concrete block would be considered "more imponderous," while the concrete block would be characterized as "heavier" or "more ponderous." Similarly, a thoughtless person might be considered "more imponderous" than a contemplative individual who is "more substantive" or "more weighty" in their thinking.