The adjective related to the word "weight" is "weighty." In the following examples, "weighty" means important; significant; serious. "The actress had a weighty role in the film." Comparative: "Her first film was rather silly, and her acting was frivolous. However, her role in her second film was weightier." Superlative: "Of all the films she has starred in, the latest film offered her her weightiest role." Please look here for all the meanings of of "weighty." http://www.thefreedictionary.com/weighty

"Weight" isn't an adjective; it's a noun. Do you mean "heavy"? If you do, then heavy, heavier, heaviest. If you meant weighty, the Neil's answer is what you need.