code>, the percentage of the dataframe's total memory footprint 
  used by each column.</p></li>
</ul><p>For a <strong>pair of dataframes</strong>, the tibble returned contains the columns: <br></p><ul><li><p><code>col_name</code>, a character vector containing column names of <code>df1</code>
  and <code>df2</code>.</p></li>
<li><p><code>size_1</code>, <code>size_2</code>, a character vector containing memory usage of each column in
  each of <code>df1</code> and <code>df2</code>.</p></li>
<li><p><code>pcn