R How to fit map created with rworldmap to plot size

I have the following map, displaying the countries covered by my sample:

require(rworldmap)
require(tidyverse)
countries <- c("AUS", "AUT", "BEL", "BGR", "BRA", "CAN", "CHE", "CHN", "CYP",
               "CZE", "DEU", "DNK", "ESP", "EST", "FIN", "FRA", "GBR", "GRC",
               "HRV", "HUN", "IDN", "IND", "IRL", "ITA", "JPN", "KOR", "LTU",
               "LUX", "LVA", "MEX", "MLT", "NLD", "NOR", "POL", "PRT", "ROU",
               "RUS", "SVK", "SVN", "SWE", "TUR", "TWN", "USA")
countries <- as.tibble(countries) %>% 
  mutate(covered = TRUE) %>% 
  rename(countries = value)

map <- joinCountryData2Map(countries, nameJoinColumn = "countries") %>% 
  subset(continent != "Antarctica")

mapCountryData( map, 
                nameColumnToPlot='covered', 
                catMethod='fixedWidth', 
                numCats=2 ,
                addLegend = FALSE,
                colourPalette = c("black", "grey"),
                borderCol = "white",
                oceanCol = 'grey',
                mapTitle = "sample coverage")

In the resulting pdf, the graph looks like this:

My question: How can I increase the size of the resulting map in the graph?

I know that in an .Rmd document, I can set fig.width of a chunk, but this increases both the size of the map and the size of the title in the resulting document. Instead, I would like to keep the size of the title fixed and only increase the size of the map relative to the title.

Thanks for your help!