library(ggplot2)
p <- ggplot(data = PlantGrowth, aes(x = group, y = weight, fill = group)) + 
    geom_boxplot()

g <- ggplotGrob(p)
library(gtable)

## Loading required package: grid

grid.draw(g)

grid.newpage()
orig <- g[["grobs"]][[8]][["grobs"]][[1]][["widths"]]
g[["grobs"]][[8]][["grobs"]][[1]][["widths"]] <- unit.c(orig[1:2], orig[3] + 
    unit(5, "mm"), orig[4:6])
grid.draw(g)