library(tidyverse)
library(maps)

temp <- maps::world.cities %>%
        ggplot() +
          geom_point(aes(x=long, y=lat, size=pop, fill=pop, color=pop), alpha=.1, show.legend = FALSE) +
          theme_void() +
          coord_equal()

ggsave( temp, file="pop.png", dpi = 300, width = 20, height = 10)