For a single dataframe, the tibble returned contains the columns:
col_name, a character vector containing column names of df1.
bytes, integer vector containing the number of bytes in each column of df1.
size, a character vector containing display-friendly memory usage of each column.
pcnt, the percentage of the dataframe's total memory footprint
used by each column.
For a pair of dataframes, the tibble returned contains the columns:
col_name, a character vector containing column names of df1
and df2.
size_1, size_2, a character vector containing memory usage of each column in
each of df1 and df2.
pcnt_1, pcnt_2, the percentage of total memory usage of each column within
each of df1 and df2.
For a grouped dataframe, the tibble returned is as for a single dataframe, but where
the first k columns are the grouping columns. There will be as many rows in the result
as there are unique combinations of the grouping variables.