Interpolation for larger areas


I am trying to interpolate a variable within my dataset over a couple regions in Brazil. However, I am having a difficult time trying to get the box around my interpolated area to expand so that I can see the entire region. I am not sure if any one has had this issue before, but I would greatly appreciate some advice. I have attached my code and a screenshot of the plot I have been generating.

BM <- CRS("+proj=aea +lat_1=-5 +lat_2=-42 +lat_0=-32 +lon_0=-60 +x_0=0 +y_0=0 +ellps=aust_SA +units=m +no_defs")
topsoil_BM <- spTransform(topsoil_sp, BM)
cerrado_BM <- spTransform(cerrado, BM)
v <- voronoi(topsoil_BM)
brazil <- aggregate(cerrado_BM)
vbrazil <- intersect(v, brazil)
spplot(vbrazil, 'd13C', col.regions=rev(get_col_regions()))

@jamiew I am having trouble reproducing this example. Can you provide a reproducible example?