I’m trying to resample my raster file to match the resolution of my second raster file as shown below;

pop2 <- resample(pop, grid, method = 'bilinear')

My raster file is population at 100M resolution and the grid is approximately 5km. When I sum the values obtained after resampling, the sum is not equal to the original and it has reduce significantly

# Before
> sum(getValues(pop), na.rm = T)
[1] 3589773

# After
> sum(getValues(pop2), na.rm = T)
[1] 31658.05

What could be happening, and is there a way to retain the sum of the values?