ggplot2: same bar widths when saving pdf

Question

I want all bars to have the same width.

My code works when not saving it to a pdf:

library(ggplot2)

dat <- as.data.frame(mtcars)
# convert rownames to a separate column
dat <- cbind(rownames(dat), data.frame(dat, row.names=NULL))
names(dat)[which(names(dat) == "rownames(dat)")] <- "type"

p <- ggplot(data=dat[1:10,]) +
  geom_col(aes(x = reorder(type, wt), y = wt), position=position_dodge2(width = 0.1, preserve = "single"), width = 0.1) +
  coord_flip()

p

But when I try to save the plot in a pdf, then the bars have slightly different widths.

pdf(paste0(path_out, 
           "test.pdf"),
    width=10, height=4)

print(p)

dev.off()

With my real data it is even worse:

I am using Windows 7 Enterprise. How can I fix this?