Change table contents' font size with formattable package from R

Question

Assuming a dataframe as follows:

df <- structure(list(week1 = c(30, 74.62, 189.93, 2394.91, 6095.8, 
2.88, 45.49), week2 = c(30, 88.4, 156.14, 2880.4, 5087.41,