How to increase the inner margin of a ggplot boxpl

I am setting up boxplots to present these in a ggplot2 facet_grid and I would like to increase the inner margin.

Unfortunately, I am not able to increase the distance to the frame of the facet.

How could I increase the inner margin (left and right) as indicated by the blue arrows?

require(ggplot2)
dat <- rbind(data.frame(approach=1,product=1,value=seq(1,20,0.5)), 
             data.frame(approach=1,product=2,value=seq(5,15,0.3)), 
             data.frame(approach=1,product=3,value=seq(5,17,0.2)), 
             data.frame(approach=2,product=1,value=seq(1,13,0.3)), 
             data.frame(approach=2,product=2,value=seq(3,18,0.5)), 
             data.frame(approach=2,product=3,value=seq(4,25,0.7)), 
             data.frame(approach=3,product=1,value=seq(1,15,0.6)), 
             data.frame(approach=3,product=2,value=seq(3,16,0.5)), 
             data.frame(approach=3,product=3,value=seq(1,10,0.1)))

gg1 <- ggplot(dat, aes(group =product, y = value)) + 
  geom_boxplot() + 
  ylab("size (cm)")+ 
  theme(panel.spacing = unit(0.1, 'lines')) + 
  theme(plot.background = element_rect(fill ="lightgrey" )) +
  scale_fill_grey(start = 0.0, end = 1) +
  theme_bw()+ 
  xlab("") +   
  facet_grid(cols=vars(approach)) +
  theme(axis.text.x = element_text(colour="black")) + 
  theme(axis.text.y=element_text(colour="black"))+ 
  theme(panel.spacing=unit(0,"lines")) + 
  guides(fill=guide_legend(title="Products")) + 
  theme(plot.background = element_rect(fill ="lightgrey" ))

gg1

Also, how would it work for a discrete scale?

require(ggplot2)
dat <- rbind(data.frame(approach=1,product=1,value=seq(1,20,0.5)), 
             data.frame(approach=1,product=2,value=seq(5,15,0.3)), 
             data.frame(approach=1,product=3,value=seq(5,17,0.2)), 
             data.frame(approach=2,product=1,value=seq(1,13,0.3)), 
             data.frame(approach=2,product=2,value=seq(3,18,0.5)), 
             data.frame(approach=2,product=3,value=seq(4,25,0.7)), 
             data.frame(approach=3,product=1,value=seq(1,15,0.6)), 
             data.frame(approach=3,product=2,value=seq(3,16,0.5)), 
             data.frame(approach=3,product=3,value=seq(1,10,0.1)))

dat$product<-as.factor(dat$product)

gg1<-ggplot(dat, aes(x =product, y = value)) +
  geom_boxplot() + 
  ylab("size (cm)")+ 
  theme(panel.spacing = unit(0.1, 'lines')) +
  theme(plot.background = element_rect(fill ="lightgrey" )) +
  scale_fill_grey(start = 0.0, end = 1) +  
  theme_bw()+ xlab("") + 
  facet_grid(cols=vars(approach)) +
  theme(axis.text.x = element_text(colour="black")) +
  theme(axis.text.y=element_text(colour="black"))+ 
  theme(panel.spacing=unit(0,"lines")) + 
  guides(fill=guide_legend(title="Products")) + 
  theme(plot.background = element_rect(fill ="lightgrey" ))

gg1

The part you are looking at is controlled from scales, not facets or theme margins.

Either of the following would work. Their results are similar in this case, since your x-values' range is in the neighbourhood of (-1, 1). More generally, look up the help file for ?expand_scale for examples of multiplicative vs. additive expansion factors

gg1 + scale_x_continuous(expand = c(0.2, 0)) # expand scales by a multiple of 20%

gg1 + scale_x_continuous(expand = c(0, 0.2)) # expand scales by an addition of 0.2