R ggplot ggsave produces different plot element sizes than simply exporting in RStudio

Question

When exporting a plot using ggsave(), nearly all elements (notably, text) of my ggplot are enlarged compared to when viewing and exporting my plot in R studio. The reason I'm using ggsave() is so that I can get the 300 dpi and 85mm width needed for publication.

How can I ensure that the ggsave() output matches what RStudio exports?

1. RStudio export (correct proportions):

2. ggsave() export (incorrect proportions):

3. My code

library(ggplot2)
library(cowplot)
library(grid)

# Make plot
fig4 <- ggplot()
# Add Landings lines
fig4 <-
  fig4 + geom_line(
    data = NorRus,
    aes(year, tonnes, color = fishing_entity),
    size = 0.75
    )
# Change colors. 
fig4 <-
  fig4 + scale_color_manual(
    values = bluesPalette,
    name = NULL
    )
# adjust y axis labels, remove scientific notation
fig4 <-
  fig4 + scale_y_continuous(
    expand = c(0,0), #removes stupid gap btwn plot & axes
    breaks = seq(0, 2500000, 500000),
    limits = c(0, 2500000),
    labels = divide1000() # divide units by 1000 w function specified above 
  )
# adjust x axis labels
fig4 <-
  fig4 + scale_x_continuous(
    expand = c(0,0), # removes stupid gap btwn plot & axes
    breaks = seq(1950, 2014, 10),
    limits = c(1950, 2014)
  )
# Add titles to axes
fig4 <- 
  fig4 + labs(
    y = Land10e3,
    x = "Year"
  )
# Adjust axis margins
fig4 <- 
  fig4 + theme(
    axis.title.y=element_text(margin=margin(0,20,0,0)),
    axis.title.x=element_text(margin=margin(20,0,0,0))
  )
# Adjust text sizes
fig4 <- 
  fig4 + theme(
    legend.title=element_text(size=10), # Legend title
    legend.text=element_text(size=8), # Legend items
    axis.title=element_text(size=10), # Axis titles
    axis.text=element_text(size=8) # Axis labels
  )
# move legend to inside
fig4 <- 
  fig4 + theme(
    legend.justification = c(1, 1), 
    legend.position = c(1, 1)
  )
# Plot
plot(fig4)

# Export plot w ggsave
ggsave(
  "R exports/fig4-ggsave.tiff",
  plot = fig4, 
  device = "tiff", 
  dpi=150, 
  width=85, 
  height=53, 
  units = "mm"
  )

I suspect my issue has something to do with element_text() being in point units, while the units I've specified with ggsave() are in mm.

These two questions are very close to helping me solve my issue, but I don't quite know how to implement them as a solution!

1. ggplots2 ggsave text size not changing
2. ggplot geom_text font size control

Answer 1

Change these arguments:

width=85, maybe to 850
height=53, maybe to 530

Manipulating with width and hight of plot will give you needed output.

Answer 2

(As per my comment on Piotr's answer): My hacky workaround is this: width=85 * (14/5), height=53 * (14/5), which is the ratio provided in my second linked question. So for now until something better comes along, I'm applying this ratio to all my ggsaves, then resizing to 85 mm in an outside program.