import pandas as pd

df_test = pd.DataFrame([
  {'dir': '/Users/uname1', 'size': 994933},
  {'dir': '/Users/uname2', 'size': 109338711},
])

def sizes(s):
  a = locale.format_string("%.1f", s['size'] / 1024.0, grouping=True) + ' KB'
  b = locale.format_string("%.1f", s['size'] / 1024.0 ** 2, grouping=True) + ' MB'
  c = locale.format_string("%.1f", s['size'] / 1024.0 ** 3, grouping=True) + ' GB'
  return a, b, c

df_test[['size_kb', 'size_mb', 'size_gb']] = df_test.apply(sizes, axis=1, result_type="expand")