Stretching axes in perspbox - 3D Plots in R

Question

I have been trying to replicate a 3D graph that I saw on a paper that I read for a class project. I recently posted a question asking about possible ways of plotting it and I received amazing support on that. However, I am still struggling to figure out how to stretch the axes to get rid of the cluttering of the values on the axes (image below).

The code that I used is as follows.

library(plot3D)
par(mar=c(1,1,1,1))

R_0 <- function(beta_s, gamma_a, alpha_a = 0.4775, alpha_u = 0.695,
                mu = 0.062, q_i = 0.078, eta_i = 0.009, eta_u = 0.05) {
  
  A <- beta_s * alpha_a / (gamma_a + mu) 
  B <- beta_s * alpha_u * gamma_a * (1 - q_i) / ((gamma_a + mu) * (eta_u + mu))
  A + B
}

beta <- gamma <- seq(0, 0.4, length.out = 100)

R <- outer(beta, gamma, R_0)

#Adding an empty perspbox

perspbox(beta, gamma, z = R, theta = -50, ticktype = "detailed",
         col.grid = "gray85", bty = "u",
         xlab = "\u03b2\u209b", ylab = "\u03b3\u2090")

pp <- persp3D(beta, gamma, z = R, theta = -50, add = TRUE)

#Horizontal plane-adding function

plane3D <- function(z, 
                    col = adjustcolor("blue", alpha.f = 0.2),
                    border = NA,
                    xlim = c(0,0.4), ylim = c(0, 0.4)) {
  dd <- expand.grid(x=xlim, y = ylim, z= z)
  rr <- with(dd, trans3D(x,y,z,pp))
  perm <- c(1,3,4,2)
  polygon(rr$x[perm], rr$y[perm], col = col, border = border)
}

#Adding planes

plane3D(1)
plane3D(2, col = adjustcolor("red", alpha.f = 0.2))

Any help would be greatly appreciated. Thanks a lot in advance!

The graph based on dcarlson's answer

Answer 1

Running your code with two changes: xlab = "\u03b2\u2090" instead of xlab = "\u03b2\u209b" and changing par(mar=c(1.5, 2, 1.5, 2)) on Mac, I get this plot:

You need to provide more information on your computer's operating system and the versions of R and plot3D that you are using.