The phrase "weighty themes" refers to important or serious topics. It suggests that the subjects being discussed or presented carry significance and require thoughtful consideration. Full definition
Critics Consensus: Of a piece with much of director Laurie Anderson's idiosyncratic output, Heart of a Dog delves into weighty themes with lyrical, haunting grace. (rottentomatoes.com)
It starts out as standard kid comedy fare a la Diary of a Wimpy Kid or How to Eat Fried Worms, but then unexpectedly snowballs into a poignant drama with weightier themes of life and death. (christianitytoday.com)
I can understand people assuming that I'm reading too much into what amounts to a super hero romp around the northern section of Second Son's Seattle, but given Sucker Punch's desire to cover weighty themes in their previous entries, I can't help but wonder if something more could have been learned from Fetch's being an addict. (darkstation.com)