tmap raster layer not rendering pixels full size

I'm trying to produce a basic heatmap and the grid pixels are rendering very small.

require(tmap)
require(dplyr)
require(stars)

d = data.frame(lon = rnorm(1e5, mean = -95.5, sd=4),
               lat = rnorm(1e5, mean = 37.5, sd=4)) |> 
  group_by(lat = round(lat), lon = round(lon)) |> 
  summarise(n = length(lat), .groups = 'drop') |> 
  st_as_sf(coords = c('lon','lat'), crs = 'WGS84') |>
  st_rasterize()

tmap_mode("view")

tm_basemap("Stamen.TonerLite") +
  tm_shape(d) +
  tm_raster("n", palette = c('white','red'), alpha=0.5)

Grateful for any suggestions what I'm doing wrong here.