Credit goes to kmuehlbauer https://github.com/pydata/xarray/issues/9946#issuecomment-2587287969 The data in the file is compressed:

This is an excerpt of the h5dump -Hp 2021-04.nc:

DATASET "t2m" {
      DATATYPE  H5T_IEEE_F32LE
      DATASPACE  SIMPLE { ( 30, 411, 791 ) / ( 30, 411, 791 ) }
      STORAGE_LAYOUT {
         CHUNKED ( 15, 206, 396 )
         SIZE 12481395 (3.126:1 COMPRESSION)
      }
      FILTERS {
         PREPROCESSING SHUFFLE
         COMPRESSION DEFLATE { LEVEL 1 }
      }
      ...