I have studied Cleopatra for many years and I have never heard any association of Cleopatra with large posteriors. There is a mythology that has grown around her, beginning with hostile Roman sources who portrayed her as a frivolous seductress, a depraved Oriental Queen who tried to sleep her way into Roman power.
Modern novels, stories, movies, etc., have exploited this mythology. The French writer Theophile Gautier portrayed her as a wicked seductress who made love with men and the murdered them. In HBO's seris Rome, Cleopatra is depicted as a young frivolous sociopath who used men dressed as deer for archery practice.
The "Real Cleopatra" was a hawk-nosed inbred heiress of the Ptolemaic dynasty who secured her authority by murdering her brother and sister. She was a Macedonian Greek who was fluent in several languages and reputedly the author of several books.